Can you tell me from memory what the compression ratio of your car's engine is? Let's say 9.8; isn't it too much? Or maybe, on the contrary, it’s not enough?
Not an easy question, because designers of spark-ignition engines [We usually say gasoline, although we know that car engines run just fine on gas. And also on alcohol - methyl or ethyl... So it’s better to put it: with spark ignition. Or Otto (named after the creator of this design, Nikolaus Otto) - in contrast to Diesel. Although it sounds strange, it is more accurate.] They strive in every possible way to increase the compression ratio. And engine creators, on the contrary, are trying to reduce it...
A peculiar characteristic of internal combustion engines, around which there are many misunderstandings. And one of the key ones is that a lot depends on the degree of compression. Although, at first glance, there is nothing simpler: the ratio of the total volume of the cylinder to the volume of the combustion chamber. Or in other words: the quotient of dividing the volume of the above-piston space in b.m.t. on him - v.m.t. That is, the geometric compression ratio shows how many times the air-fuel mixture (air in the diesel cylinders) is compressed when the piston moves from ground level. to e.m.t. Geometric; and in life, naturally, things don’t always turn out the way they do in geometry...
Volumes of a 4-stroke piston engine: Vk – volume of the combustion chamber; Vp – cylinder working volume; Vo – total volume of the cylinder; TDC – top dead center; BDC – bottom dead center. Onward and higher
At the dawn of motoring, the compression ratio of Otto engines (and in fact, they didn’t know any others 100 years ago) was made low - 4-5. So that when working on low-octane gasoline (drive as best you can), detonation does not occur [Who hasn't heard detonation sounds in the cylinders? As they say, “fingers are tapping.” If the compression ratio is too high (in terms of fuel quality), the combustion of the air-fuel mixture after it is ignited by a spark is disrupted. It becomes explosive, shock waves appear in the combustion chamber, which will harm the engine.]. Let’s say, with a cylinder working volume of 400 “cubes”, the volume of the combustion chamber is 100 milliliters. That is, the geometric compression ratio of our engine
e = (400+100)/100 = 5.
If the volume of the combustion chamber is reduced - all other things being equal - to 40 cm 3 (technically not difficult), then the compression ratio will increase to
e = (400+40)/40 = 11.
Great - so what? And the fact that thermal efficiency engine will increase almost 1.3 times. And if a 6-cylinder 2.4-liter engine develops a power of 100 hp with a compression ratio of 5, then with a compression ratio of 11 it will increase to almost 130. And with constant fuel consumption! In other words, fuel consumption per 1 hp. per hour is reduced by 22.7%.
Short stroke 3.8 liter Porsche 911 engine with 11.8 compression ratio! The volume of the combustion chamber is so small (59 cm3) that it is difficult to make recesses in the piston bottom for the valve heads Amazing results – using the simplest means. Is it too good to be true? There is no mysticism: the higher the compression ratio, the lower the temperature of the exhaust gases going to the exhaust. At e= 11 we simply heat the atmosphere noticeably less than with degree 5; that's all.
Basics of heating engineering
Car engines are a type of heat engine that obeys the laws of thermodynamics. Back in the 1st half of the 19th century. the remarkable French physicist and engineer Sadi Carnot laid the foundations of the theory of heat engines - including internal combustion engines. So, according to Carnot, efficiency of an internal combustion engine, the greater the difference between the temperature of the gases (working fluid) at the end of combustion of the air-fuel mixture and their temperature at the outlet. And the temperature difference depends on e- or rather, on the degree of expansion of the working gases in the cylinders.
Sadi Carnot (1796-1832) Yes, there is a nuance here: according to Carnot, for thermal efficiency. It is not the degree of compression that is important, but the degree of expansion. The more hot gases expand during the working stroke, the lower their temperature drops - naturally. It’s just that in conventional internal combustion engine designs. the degree of expansion geometrically coincides with the degree of compression; That's what we're used to talking about. Moreover, detonation depends precisely on e– that is, from compression. The more the air-fuel mixture is compressed in the cylinders of the Otto engine [Exactly Otto, diesel engines don’t know detonation. Why is a separate conversation.], the higher the pressure and temperature at the time of spark formation, the more likely the occurrence of shock waves in the combustion chamber.
Explosive combustion, detonation. This limits the degree of compression, but the degree of expansion of the working gases has nothing to do with it. Now, if you somehow separate one degree from another - in order to achieve a strong expansion of the working gases with moderate compression...
Five-stroke cycle
Pourquoi would not pas; After all, the so-called Atkinson/Miller 5-stroke cycle has been known for more than half a century. It just sets the degree of compression and the degree of expansion on different sides.
Imagine that your 1.5-liter 16-valve VAZ-2112 intake does not end at 36° after ground level. (according to the angle of rotation of the crankshaft), and very late - by 81°. That is, at 3 thousand revolutions the piston is on its way to the TDC. displaces part of the air-fuel mixture through the open valves back into the intake manifold (don’t worry, it won’t go to waste there). In other words, the compression stroke begins only somewhere around 75° after bpm, and before that a kind of reverse displacement stroke of the mixture takes place.
There are now not 4, but 5 strokes: intake, reverse displacement, compression, power stroke, exhaust. At first glance, this is an idiotic scheme: why push the mixture back and forth? At first glance, the Sun also revolves around the Earth... Follow my hands: let’s say that 20% of the air-fuel mixture that has already entered the cylinder is forced back, and only 80% is compressed. And let it be geometric e equal to 13 - exceptionally high for Otto. However, the actual compression ratio is much lower: with 20 percent reverse displacement of the mixture, it is equal to 10.6. Q.E.D.
For a design with a real compression ratio of 10.6 (quite acceptable for commercial gasoline), the expansion ratio of the working gases is 13. Thermal efficiency. the engine is in fact 1.0518 times higher than its actual compression ratio; not much, but engine builders have been fighting for years to achieve 5 percent fuel savings. Passenger car engines are already running on a 5-stroke cycle. Take Toyota's 1.5-liter 1NZ-FXE four (for the Prius) or Ford's 2.26-liter (for the Escape hybrid). It seems like a brilliant solution, but there is a downside to the coin.
Toyota “four” 1NZ-FXE: also a 5-stroke cycle. It is noticeable to the eye how much wider the profile of the intake cam is than the exhaust cam: extremely late closing of the intake valves Geometric e(the degree of expansion of working gases) for 1NZ-FXE is 13, the actual compression ratio is about 10.5. The sad thing is that due to reverse displacement of the mixture, the 1.5-liter engine drops in power and power to approximately 1.2-liter; we win in thermal efficiency – at the cost of losing real displacement. So on the one hand - on the other hand.
Moreover, an engine with late closing of the intake valves does not pull “at the bottom” at all. Therefore, the 5-stroke cycle is suitable in “hybrid” power units, where the traction electric motor takes on the load at the lowest speeds. And then it picks up the engine; one way or another, the 5-stroke cycle allows you to increase the degree of expansion of the working gases and thermal efficiency. engine.
In a Honda engine operating on a 5-stroke cycle, part of the air-fuel mixture is forced out by the piston back into intake ports 1 – intake; 2 – reverse emission of the air-fuel mixture; 3 – fifth bar: compression. But supercharging, on the contrary, forces you to lower the compression ratio. When the air-fuel mixture is supplied under excess pressure, the actual compression in the cylinders turns out to be too high - even with moderate geometric e. We have to retreat; hence the reduction in thermal efficiency. and increased gasoline consumption for supercharged engines, if special fuel is not used.
On alcohol
The higher the octane number of gasoline, the higher the permissible (according to detonation conditions) compression ratio, the more efficiently the engine operates. Well, not with gasoline alone... Exceptionally high e allows gas - oil or natural - as fuel. Without supercharging 13-14 is not a problem, with a compressor – 10-11. Hydrogen is also resistant to detonation. And also alcohol - methyl or ethyl: amazing anti-knock qualities. In addition, alcohol has a high heat of vaporization; evaporating, it greatly cools the air-fuel mixture (and at the same time the surface of the combustion chamber). The cold mixture is denser, and much more of it, by weight, enters the cylinder; the actual filling factor turns out to be higher. , power. That’s what they say: the “compressor” effect of alcohol fuel.
Power, thermal efficiency - all the pleasures at once. In addition, ethyl (drinking!) alcohol is also environmentally friendly; what more could you want? True, the consumption of alcohol fuel in liters turns out to be much higher than gasoline, since the calorific value of methanol and ethanol is low. Like vodka and “sushnyak”; It makes no sense to equate liter to liter here. But in energy equivalent, alcohol is noticeably more efficient than gasoline - due to the high degree of compression (expansion). So in the future - alcohol fuel, pure or mixed with gasoline. Let's say E85: 85% ethanol and 15% gasoline. And in 25 years, oil will lose its importance in the world...
Truth in moderation
In the future, in the meantime, increasing the compression ratio of the VAZ 16-valve from 10.5 to 11.5 - on 92 gasoline from a local gas station - oh, how difficult it is. Let's say, apply gasoline injection directly into the combustion chambers - instead of the intake channels. The evaporation of gasoline not at the inlet, but in the cylinders - the same “compressor” effect. Or organize 2-spark ignition - with 2 spark plugs per cylinder; gives something. And also install exhaust valves with internal (sodium) cooling; hot plates provoke detonation. Clean the surface of the combustion chamber from carbon deposits and polish it.
The configuration of the combustion chamber affects the speed of the vortex movement of the air-fuel mixture. There are many ways to combat detonation - good and different.
To what level does it make sense to raise e Otto engine? Here's what it's all about: thermal efficiency. increases with increasing degree of compression (expansion!), but not linearly. That is, the increase in efficiency slows down: if from 5 to 10 it increases by 1.265 times, then from 10 to 20 – only by 1.157 times. But side problems quickly accumulate, which are best avoided. Therefore, a compression ratio of 13-14 is a reasonable compromise that should be strived for. Just leave the final decision to the design engineers; they know better.
I think many people ask this question in the vast expanses of Russian roads. What kind of gasoline is better to pour into your iron horse, 92 or 95? Is there a critical difference between them, and what will happen if you use 92 gasoline instead of 95? After all, it is about 5 - 10% cheaper, and therefore there will be real savings from each tank! BUT is it worth doing this and isn’t it dangerous for your power unit? Let’s break it down piece by piece, there will be a video version and voting at the end...
At the very beginning, I propose to think about what these numbers are, 80, 92, 95, and in Soviet times also 93? Ever wondered? It's all just the octane number. Then what is it? Read on.
Octane number of gasoline
The octane number of gasoline is an indicator characterizing the detonation resistance of the fuel, that is, the amount of the fuel’s ability to resist self-ignition during compression for internal combustion engines. That is, in simple words, the higher the “octane level” of the fuel, the less likely it is for the fuel to spontaneously ignite during compression. In such a study, fuel levels are differentiated according to this indicator. Research is carried out on a single-cylinder installation with a variable level of fuel compression (they are called UIT-65 or UIT-85).
The units operate at 600 rpm, air and mixture are 52 degrees Celsius, and the ignition timing is about 13 degrees. After such tests, the RON (research octane number) is derived. This study should show how gasoline will behave under minimal and medium loads.
At maximum fuel loads, there is another experiment that deduces (ROM - motor octane number). Tests are carried out on this single-cylinder installation, only the speed is 900 rpm, the air and mixture temperature is 149 degrees Celsius. NMO has a lower value than OCHI. During the experiment, the level of maximum loads is displayed, for example, during throttle acceleration or when driving uphill.
Now I think it has become at least a little clear what it is. And how it is defined.
Now let's get back to the choice - 92 or 95. Any type, be it 92 or 95, or even 80. When processed at the factory, it does not have such a final octane number. With direct distillation of oil, it turns out only 42 - 58. That is, very low quality. “How can this be,” you ask? Is it really impossible to distill immediately with a high rate? It is possible, but it is very expensive. A liter of such fuel would cost several times more than those currently on the market. The production of such fuel is called catalytic reforming. Only 40 - 50% of the total mass is produced in this way and mainly in Western countries. In Russia, much less gasoline is produced this way. The second production technology, which is less expensive, is called catalytic cracking or hydrocracking. Gasoline with this treatment has an octane number of only 82-85. In order to bring it to the desired level, you need to add special additives to it.
Gasoline additives
1) Additives based on metal-containing compounds. For example, on tetraethyl lead. Conventionally, they are called leaded gasoline. Very efficient, they make the fuel work, as they say. But also very harmful. As the name tetraethyl lead suggests, it contains a metal called “lead.” When burned, it forms gaseous lead compounds in the air, which is very harmful, settles in the lungs, developing complex diseases, such as “CANCER”. Therefore, these types are now banned all over the world. In the USSR there was a grade called AI-93, which was based on tetraethyl lead. We can conditionally call this fuel obsolete and harmful.
2) More advanced and safer based on ferrocene, nickel, manganese, but monomethylaniline (MMNA) is most often used, its octane number reaches 278 points. These additives are mixed directly with gasoline, bringing the mixture to the desired consistency. But such additives are also not ideal; they form deposits on pistons, spark plugs, clog catalysts and all kinds of sensors. Therefore, sooner or later, such fuel will clog the engine, in the literal sense of the word.
3) Latest and the most perfect are ethers and alcohols. The most environmentally friendly and do not harm the environment. But there are also disadvantages of such fuel, this is the low octane number of alcohols and ethers, the maximum value is 120 points. Therefore, the fuel requires quite a lot of such additives, about 10 - 20%. Another drawback is the aggressiveness of alcohol and ether additives; with high contents, they quickly corrode rubber and plastic pipes and sensors. Therefore, such additives are limited to 15% of the total fuel level.
Compression ratio and the modern car
Actually, why I started talking about the octane number and additives, because it is necessary to take into account the self-ignition of the fuel or the so-called detonation in modern units.
The fact is that manufacturers, in order to increase power and reduce fuel consumption, slightly increase the compression ratio in the engine cylinders.
Here's some useful information:
- For compression ratios up to 10.5 and below, the octane number of gasoline is AI - 92 (we do not take into account TURBO engine options).
- From the 10.5 to 12 mark - fill in fuel not lower than AI - 95!
- If the compression ratio is 12 or higher, then it is recommended to fill in at least AI - 98
- Of course, there are also very rare gasolines, such as AI-102 and AI-109, for which the compression ratio is 14 and 16, respectively.
So what will happen IN THEORY , if we pour 92 gasoline into an engine that is designed for 95? YES, everything is simple, fuel from a high compression ratio will self-ignite, “mini-explosions” will occur - that is, the destructive effect of detonation will manifest itself!
Why is detonation dangerous? Yes, everything is simple, burnout of the gasket between the head of the block and the block itself, destruction of the rings (both compression and oil control), burnout of the pistons, etc.
BUT it’s like I wrote above - IT'S ALL IN THEORY ! ESPECIALLY IN RUSSIA! Why am I saying this? Many manufacturers have realized that it is VERY DIFFICULT to find high-quality gasoline (and now we are talking about the 95 version), if possible, even in metropolitan regions (I’m already silent about small cities). Gasoline is often bottlenecked so that it is impossible to achieve an octane rating of 95. I remember a couple of years ago, I read an article with an experiment - where in the capital they took samples from a large number of gas stations, and only in 20 - 25% of cases the gasoline was close to the standards, the rest were far from the figure 95 and even 92. Just think about it! How can you check the quality yourself? That's right - NO way.
So if you fill in such low-quality fuel, will the engine immediately shut down? Straightaway? Not certainly in that way. Cars are smart now, and to prevent your engine from going haywire, a knock sensor was invented; it allows the engine to operate with a different octane number. It monitors the mechanical vibrations of the engine block, converts them into electrical impulses and constantly.
If the pulses “go beyond the normal state,” then the ECU makes a decision to adjust the ignition angle and the quality of the fuel mixture. Thus, a modern engine designed for 95 gasoline will run smoothly even on 92.
However! Such work will be successful at low and medium speeds; at high speeds (almost maximum), the knock sensor does not work so effectively, so “frying” with a low-octane mixture is UNDESIRABLE!
Let's summarize.
What happens if you fill in 92 instead of 95?
In fact, the difference between 92 and 95 gasoline is minimal, only “3 numbers.” If you refuel in a company that guarantees you exactly “hard indicators”, that is, “92 is 92”, and “95 is 95” and YOU WILL BE SURE OF THIS. The difference will appear for your engine at high speeds, and not in a significant (up to 2 - 3%) loss of power, and fuel consumption will also increase by this percentage.
And what’s most interesting is that if you don’t often spin your power unit to 5000 - 7000 rpm, but move from 2000 to 4000, then 92 will not give you any negative aspects. Still, the electronics will regulate everything itself.
Prejudice - there is no such thing. Burnout of valves was typical for leaded types that had metal additives. High-octane leaded gasoline could harm an engine configured to use AI-76 (and it did not have electronic correction of the ignition angle and fuel injection). But now there is simply no such danger, because such fuel has long been prohibited.
BUT IDEAL! You need to fill with the exact fuel recommended by your manufacturer. After all, if suddenly a new engine breaks down, and it turns out that the breakdown is related to gasoline, then you will end up with very expensive repairs, AND AT YOUR OWN EXPENSE. Saving 10% on gasoline will hurt you.
Everyone knows that in gasoline piston internal combustion engines, the air-fuel mixture is compressed before ignition. A similar operating cycle of diesel engines differs only in that air is compressed without fuel. One of the most important characteristics of both internal combustion engines is the compression ratio. It shows how many times the volume of space above the piston bottom changes as it passes from bottom dead center to top.
Sometimes this indicator is confused with compression, despite the fact that the difference between them is huge. After all, the characteristics mentioned above, although interconnected, are essentially completely different. As even their size indicates. The compression ratio is a ratio, for example 10:1 or simply 10, and has no units of measurement. That is, it is measured in “times”. Compression shows the maximum pressure of the mixture in the cylinder before ignition and is measured in kg/cm2. Thus, the compression of an internal combustion engine with a compression ratio of 10:1 should be no more than 15.8 kg/cm 2 . You can say what the compression ratio is in another way. This is the ratio of the volume above the piston at bottom dead center to the volume of the combustion chamber. The combustion chamber is the space above the piston that has reached top dead center.
Calculation of compression ratio
You can calculate the compression ratio of an internal combustion engine if you perform the calculation using the formula ξ = (V p + V s)/ V s; where V r is the working volume of the cylinder, V c is the volume of the combustion chamber. From the formula it is clear that the compression ratio can be increased by reducing the volume of the combustion chamber. Or by increasing the working volume of the cylinder without changing the combustion chamber. V r is much greater than V c. Therefore, we can assume that ξ is directly proportional to the working volume and is inversely related to the volume of the combustion chamber.
The working volume of a cylinder can be calculated by knowing the cylinder diameter - D and the piston stroke - S. The formula for calculating it looks like this: V р = (π * D 2 /4) * S. 
The volume of the combustion chamber, due to its complex shape, is usually not calculated, but measured. This can be done by pouring liquid into it. You can determine the volume that fits in the liquid chamber using measuring cups or scales. It is convenient to use water for weighing, since its specific gravity is 1 g per cm 3. This means that its weight in grams will also show its volume in cubic meters. cm.
The influence of compression ratio on motor characteristics
The higher the compression ratio, the greater the compression of the internal combustion engine and its power (all other things being equal). By increasing the compression ratio, we also help increase engine efficiency by reducing specific fuel consumption. The compression ratio of the internal combustion engine determines the octane number of gasoline used to operate the engine. Thus, low-octane fuel will cause a higher value of this coefficient. An excessively high octane number of fuel will not allow a power unit, the compression of which is low, to develop full power.
Initial data
Octane number of fuel used for gasoline engines with various compression ratios.
Aligning the interface between the head and the block by cutting off the metal layer leads to a reduction in the combustion chamber of the motor. This causes the compression ratio to increase by an average of 0.1 when the head thickness decreases by 0.25 mm. Having this data at your disposal, you can determine whether it will exceed the permissible limits after repairing the cylinder head. And shouldn't measures be taken to reduce it? Experience shows that when a layer of less than 0.3 mm is removed, the consequences may not be compensated.
Why is it necessary to change the compression ratio?
The need to change this parameter of the internal combustion engine occurs quite rarely. We can list just a few reasons to do this.
How can you change the compression ratio?
Magnification methods:
- Boring of cylinders and installation of larger pistons.
- Reducing the volume of combustion chambers. It is carried out by removing the metal layer from the side of the plane where the head meets the block. Due to the softness of aluminum, this operation is best done on a milling or planing machine. A grinding machine should not be used, as its stone will constantly become clogged with ductile metal.
Ways to reduce:
- Removing a layer of metal from the piston bottom (this is usually done on a lathe).
- Installation of a duralumin spacer between the head and the cylinder block between two gaskets.
Relationship between compression ratio and compression
Knowing the value of the compression ratio, you can calculate what compression should be in the engine. However, the reverse assessment will not correspond to reality. Since compression also depends on the wear of parts of the cylinder-piston group and the gas distribution mechanism. Low engine compression often indicates significant engine wear and the need for repair, and not a low compression ratio.
Turbocharged engines
Air is pumped into the cylinders of a turbocharged engine by a compressor at a pressure slightly greater than atmospheric pressure. This means that to determine the compression ratio of such a motor, you need to multiply the value that you get as a result of the calculation using the formula by the turbocharger coefficient. Turbocharged gasoline engines operate on fuel with an octane rating higher than gasoline, which is consumed by the same engines without turbines, precisely because their ξ coefficient is higher.
The compression ratio is a calculated value that shows the change in volume before and after compression. And compression is a value that is measured realistically. During the compression process, not only volume and pressure change, but also temperature, so in a working engine the compression is usually slightly higher. It is also affected by possible leaks of valves, gaskets, rings, etc. The engine manual usually contains an indication of the minimum compression value at which it is allowed to drive.
Basic concept
It is important to know what compression ratio is optimal for the engine. This is a difficult question, because spark ignition engine developers are aiming to increase this figure. And if the engine runs on compression ignition, then it is best to lower this parameter. It is the compression ratio that is the characteristic of internal combustion engines that causes the greatest number of misconceptions.
The most common misconception is that a lot depends on the degree of compression. However, everything is simple here - this indicator is a reflection of the ratio of the volume of the cylinder to a similar parameter of the combustion chamber, and if it is different, then it is equal to the quotient of the volume of space above the piston divided by the volume of the combustion chamber. It turns out that the compression ratio in geometric terms is a reflection of how many times the volume of the air-fuel mixture in the engine cylinders is reduced as the piston moves from bottom to top dead center. Of course, in life everything is rarely the same as expressed in theory.
How does it all work?
At the dawn of motoring, the engine compression ratio was low - 4-5, so that detonation would not occur as a result of running on gasoline with a low octane number. For example, with a 400 cc cylinder, the volume of the combustion chamber will be 100 ml. It turns out that for such an engine the compression ratio will be equal to: e = (400 + 100) : 100 = 5. If the volume of the fuel chamber is reduced to 40 cubic centimeters, the compression ratio will increase: e = (400 + 40) : 40 = 11 .
What will be the result? Increase in engine thermal efficiency by almost 30%. Provided that a 2.4-liter engine with 6 cylinders with a compression ratio of 5 reaches a power of 100 horsepower, then with a compression ratio of 11 it will be equal to almost 130 horsepower. With. At the same time, fuel is consumed in the same volume. It turns out that per horsepower per hour we can talk about reducing fuel consumption by 22.7%.
This result is amazing, and the means to achieve it are incredibly simple. This is not mysticism. The higher the compression ratio of the engine, the lower the temperature of the gases that go to the exhaust after exhaust.
Basics of heating engineering
Car engines are a type of thermal units that obey the laws of thermodynamics. Physicist Sadi Carnot proposed the first foundations of the theory of heat engines in the first half of the nineteenth century. In accordance with his theory, the efficiency of such an engine is higher, the greater the difference between the temperature of the gases at the end of combustion of the fuel-air mixture and the temperature at the outlet. This difference is most influenced by the degree of expansion of the working gases inside the cylinders. There is an important point here, in accordance with his theory, what is more important for thermal efficiency is not the compression ratio, but the expansion ratio. The stronger the expansion of hot gases during the working stroke, the lower their temperature becomes, which is quite natural. In engines with a conventional design, the compression ratio fully corresponds to the expansion ratio. That is why many do not share these terms. And the compression ratio and compression together cause detonation. The stronger the compression of the air-fuel mixture in the engine cylinders, the higher the temperature and pressure at the moment of spark formation, the higher the likelihood of shock waves appearing in the detonation and combustion chamber. It is this that reduces the compression ratio, but it has nothing to do with the degree of gas expansion during operation.
Five-stroke cycle
There is a five-stroke cycle that is designed to dilute the compression ratio and expansion ratio. For example, the compression ratio of the VAZ 2112 begins to work only at 75 degrees above the lower meter point, and here there is a certain cycle of displacement of the mixture. Now there are 5 strokes: injection, back displacement, compression, power stroke and exhaust. A question arises related to the need to drive the mixture in both directions. For example, 20% of the mixture will be forced back, and 80% will be compressed as expected. Even under these conditions, the actual compression ratio and compression are 10.6.
Practical significance
If the design has a real indicator of 10.6, and the expansion of the working gases is 13, then this is quite normal. In this case, in fact, the thermal efficiency of the engine is 1.0518 times higher than that of the compression ratio. This is not enough, but engine designers have been trying for years to change the situation so as to achieve these 5% fuel savings. Passenger car engines operate on a 5-stroke cycle.
This solution seems brilliant, but there is a drawback. The geometric indicator of the degree of expansion of the working gases is 13, and for the real compression ratio - 10.5. The process of displacing the mixture back makes the 1.5 liter engine 1.2 liter in terms of power and torque. The result of this is an increase in thermal efficiency due to the loss of displacement. “At lower levels” the engine with delayed closing of the intake valves does not perform well. The five-stroke cycle is appropriate to use on cars with hybrid units, where the traction electric motor takes on the load at the lowest speeds. Next, the internal combustion engine comes into operation. And here it is not so important what the compression ratio of the engine is, what is most important is the degree of expansion of the gases during operation.
Conclusion
Due to supercharging, the compression ratio needs to be lowered. In the process of supplying the air-fuel mixture with excess pressure, it turns out that there is increased actual compression in the cylinders. Therefore, it is necessary to retreat. That is why there is a need to reduce thermal efficiency and increase fuel consumption if special-purpose fuel is not used.
I think many people ask this question in the vast expanses of Russian roads. What kind of gasoline is better to pour into your iron horse, 92 or 95? Is there a critical difference between them, and what will happen if you use 92 gasoline instead of 95? After all, it is about 5 - 10% cheaper, and therefore there will be real savings from each tank! BUT is it worth doing this and isn’t it dangerous for your power unit? Let’s break it down piece by piece, there will be a video version and a vote at the end.
At the very beginning, I propose to think about what these numbers are, 80, 92, 95, and in Soviet times also 93? Ever wondered? It's all just the octane number. Then what is it? Read on.
Octane number of gasoline
The octane number of gasoline is an indicator characterizing the detonation resistance of the fuel, that is, the amount of the fuel’s ability to resist self-ignition during compression for internal combustion engines. That is, in simple words, the higher the “octane level” of the fuel, the less likely it is for the fuel to spontaneously ignite during compression. In such a study, fuel levels are differentiated according to this indicator. Research is carried out on a single-cylinder installation with a variable level of fuel compression (they are called UIT-65 or UIT-85).
The units operate at 600 rpm, air and mixture are 52 degrees Celsius, and the ignition timing is about 13 degrees. After such tests, the RON (research octane number) is derived. This study should show how gasoline will behave under minimal and medium loads.
At maximum fuel loads, there is another experiment that deduces (ROM - motor octane number). Tests are carried out on this single-cylinder installation, only the speed is 900 rpm, the air and mixture temperature is 149 degrees Celsius. NMO has a lower value than OCHI. During the experiment, the level of maximum loads is displayed, for example, during throttle acceleration or when driving uphill.
Now I think it has become at least a little clear what it is. And how it is defined.
Now let's get back to the choice - 92 or 95. Any type, be it 92 or 95, or even 80. When processed at the factory, it does not have such a final octane number. With direct distillation of oil, it turns out only 42 - 58. That is, very low quality. “How can this be,” you ask? Is it really impossible to distill immediately with a high rate? It is possible, but it is very expensive. A liter of such fuel would cost several times more than those currently on the market. The production of such fuel is called catalytic reforming. Only 40 - 50% of the total mass is produced in this way and mainly in Western countries. In Russia, much less gasoline is produced this way. The second production technology, which is less expensive, is called catalytic cracking or hydrocracking. Gasoline with this treatment has an octane number of only 82-85. In order to bring it to the desired level, you need to add special additives to it.
Gasoline additives
1) Additives based on metal-containing compounds. For example, on tetraethyl lead. Conventionally, they are called leaded gasoline. Very efficient, they make the fuel work, as they say. But also very harmful. As the name tetraethyl lead suggests, it contains a metal called “lead.” When burned, it forms gaseous lead compounds in the air, which is very harmful, settles in the lungs, developing complex diseases, such as “CANCER”. Therefore, these types are now banned all over the world. In the USSR there was a grade called AI-93, which was based on tetraethyl lead. We can conditionally call this fuel obsolete and harmful.
2) More advanced and safer ones are based on ferrocene, nickel, manganese, but most often they use monomethylaniline (MMNA), its octane number reaches 278 points. These additives are mixed directly with gasoline, bringing the mixture to the desired consistency. But such additives are also not ideal; they form deposits on pistons, spark plugs, clog catalysts and all kinds of sensors. Therefore, sooner or later, such fuel will clog the engine, in the literal sense of the word.
3) The last and most perfect are ethers and alcohols. The most environmentally friendly and do not harm the environment. But there are also disadvantages of such fuel, this is the low octane number of alcohols and ethers, the maximum value is 120 points. Therefore, the fuel requires quite a lot of such additives, about 10 - 20%. Another drawback is the aggressiveness of alcohol and ether additives; with high contents, they quickly corrode rubber and plastic pipes and sensors. Therefore, such additives are limited to 15% of the total fuel level.
Compression ratio and the modern car
Actually, why I started talking about the octane number and additives, because it is necessary to take into account the self-ignition of the fuel or the so-called detonation in modern units.
The fact is that manufacturers, in order to increase power and reduce fuel consumption, slightly increase the compression ratio in the engine cylinders.
Here's some useful information:
For compression ratios up to 10.5 and below, the octane number of gasoline is AI - 92 (we do not take into account TURBO engine options).
From the 10.5 to 12 mark - fill in fuel not lower than AI - 95!
Of course, there are also very rare gasolines, such as AI-102 and AI-109, for which the compression ratio is 14 and 16, respectively.
So what will happen, IN THEORY, if we pour 92 gasoline into an engine that is designed for 95? YES, everything is simple, fuel from a high compression ratio will self-ignite, “mini-explosions” will occur - that is, the destructive effect of detonation will manifest itself!
Why is detonation dangerous? Yes, everything is simple, burnout of the gasket between the head of the block and the block itself, destruction of the rings (both compression and oil control), burnout of the pistons, etc.
BUT it’s like I wrote above – ALL THIS IS IN THEORY! ESPECIALLY IN RUSSIA! Why am I saying this? Many manufacturers have realized that it is VERY DIFFICULT to find high-quality gasoline (and now we are talking about the 95 version), if possible, even in metropolitan regions (I’m already silent about small cities). Gasoline is often bottlenecked so that it is impossible to achieve an octane rating of 95. I remember a couple of years ago, I read an article with an experiment - where in the capital they took samples from a large number of gas stations, and only in 20 - 25% of cases the gasoline was close to the standards, the rest were far from the figure 95 and even 92. Just think about it! How can you check the quality yourself? That's right - NO way.
So if you fill in such low-quality fuel, will the engine immediately shut down? Straightaway? Not certainly in that way. Cars are smart now, and to prevent your engine from going haywire, a knock sensor was invented; it allows the engine to operate with a different octane number. It monitors the mechanical vibrations of the engine block, converts them into electrical impulses and constantly sends them to the ECU.
If the pulses “go beyond the normal state,” then the ECU makes a decision to adjust the ignition angle and the quality of the fuel mixture. Thus, a modern engine designed for 95 gasoline will run smoothly even on 92.
However! Such work will be successful at low and medium speeds; at high speeds (almost maximum), the knock sensor does not work so effectively, so “frying” with a low-octane mixture is UNDESIRABLE!
Let's summarize.
What happens if you fill in 92 instead of 95?
In fact, the difference between 92 and 95 gasoline is minimal, only “3 numbers.” If you refuel in a company that guarantees you exactly “hard indicators”, that is, “92 is 92”, and “95 is 95” and YOU WILL BE SURE OF THIS. The difference will appear for your engine at high speeds, and not in a significant (up to 2 - 3%) loss of power, and fuel consumption will also increase by this percentage.
And what’s most interesting is that if you don’t often spin your power unit to 5000 - 7000 rpm, but move from 2000 to 4000, then 92 will not give you any negative aspects. Still, the electronics will regulate everything itself.
There are prejudices - that valves can burn out, there is no such thing. Burnout of valves was typical for leaded types that had metal additives. High-octane leaded gasoline could harm an engine configured to use AI-76 (and it did not have electronic correction of the ignition angle and fuel injection). But now there is simply no such danger, because such fuel has long been prohibited.
BUT IDEAL! You need to fill with the exact fuel recommended by your manufacturer. After all, if suddenly a new engine breaks down, and it turns out that the breakdown is related to gasoline, then you will end up with very expensive repairs, AND AT YOUR OWN EXPENSE. Saving 10% on gasoline will hurt you.
What final result you want to get - to each his own, if your engine is not designed for the 92nd, then you shouldn’t drain it! Still, it can be fraught! However, if you fill it up, a modern engine will automatically adjust the ignition angles and you may not even feel the fuel change (that is, you can drive the 92 without revving your engine to the maximum). But if a breakdown occurs, and the warranty reveals that the wrong fuel was filled, REPAIRS WILL BE AT YOUR EXPENSE! And this, for sure, is not worth the 2–3 rubles saved per liter.
Now let's watch the detailed video version.
