What Compression Ratio Means and Why It Matters
Blog

What Compression Ratio Means and Why It Matters

An internal combustion engine is a type of power unit that uses the energy released as a result of the combustion of fuel (gasoline, gas or diesel fuel). The cylinder-piston mechanism, through the crank-connecting rod, converts the reciprocating movements into rotational ones.

The power of the power unit depends on many factors, and one of them is the compression ratio. Let's consider what it is, how it affects the power characteristics of a car, how to change this parameter, and also how CC differs from compression.

What Compression Ratio Means and Why It Matters

Compression ratio formula (piston engine)

First, briefly about the compression ratio itself. In order for the air-fuel mixture not only to ignite, but to explode, it must be compressed. Only in this case is a shock generated, which will move the piston inside the cylinder.

A piston engine is an internal combustion engine, based on which the process of obtaining mechanical action is achieved by expanding the working volume of fuel. When fuel is burned, the released volume of gases push the pistons and due to this the crankshaft rotates. This is the most common type of internal combustion engine.

What Compression Ratio Means and Why It Matters

The compression ratio is calculated using the following formula: CR = (V + C) / C

V - working volume of the cylinder

C is the volume of the combustion chamber.

These engines consist of multiple cylinders in which pistons compress fuel in a combustion chamber. The compression ratio is determined by the change in the volume of space inside the cylinder in the extreme positions of the piston. That is, the ratio of the volume of space when the fuel is injected and the volume when it ignites in the combustion chamber. The space between the bottom and top dead center of the piston is called the working volume. The space in the cylinder with the piston at top dead center is called the compression space.

Compression ratio formula (rotary piston engine)

A rotary piston engine is an engine in which the role of a piston is assigned to a trihedral rotor that performs complex movements inside the working cavity. Now such engines are used mainly in Mazda cars.

What Compression Ratio Means and Why It Matters

For these engines, the compression ratio is defined as the ratio of the maximum to the minimum volume of the working space when the piston rotates.

CR = V1/V2

V1 - maximum working space

V2 is the minimum amount of working space.

Influence of compression ratio

The CC formula will show how many times the next portion of fuel will be compressed in the cylinder. This parameter affects how well the fuel burns, and the content of harmful substances in the exhaust depends on this in turn.

There are engines that change the compression ratio depending on the situation. They operate with a high compression ratio at low loads and a low compression ratio at high loads.

At high loads, the compression ratio needs to be low to prevent knocking. At low loads, it is recommended that it be higher for maximum ICE efficiency. In a standard piston engine, the compression ratio does not change and is optimal for all modes.

What Compression Ratio Means and Why It Matters

The higher the compression ratio, the stronger the compression of the mixture before ignition. The compression ratio affects:

  • The efficiency of the engine, its power and torque;
  • emissions;
  • fuel consumption.

Is it possible to increase the compression ratio

This procedure is used when tuning a car engine. Forcing is achieved by changing the volume of the incoming portion of fuel. Before performing this modernization, it should be borne in mind that with an increase in the power of the unit, the load on the parts not only of the internal combustion engine itself, but also of other systems, for example, the transmission and chassis, will also increase.

What Compression Ratio Means and Why It Matters

It is worth considering that the procedure is expensive, and in the case of alteration of already sufficiently powerful units, the increase in horsepower may be insignificant. There are several ways to increase the compression ratio in the cylinders below.

Cylinder boring

A more favorable time for this procedure is a major overhaul of the motor. All the same, the cylinder block will need to be disassembled, so it will be cheaper to perform these two tasks simultaneously.

What Compression Ratio Means and Why It Matters

When boring the cylinders, the engine volume will increase, and this will also require the installation of pistons and rings of a larger diameter. Some people choose repair pistons or rings, but for boosting it is better to use analogues for units with a large volume, set at the factory.

Boring must be carried out by a specialist using special equipment. This is the only way to achieve perfectly uniform cylinder sizes.

Finalization of the cylinder head

The second way to increase the compression ratio is to cut the bottom of the cylinder head with a milling cutter. In this case, the volume of the cylinders remains the same, but the space above the piston changes. The edge is removed within the limits of the motor design. This procedure should also be performed by a specialist who is already engaged in this kind of modification of motors.

In this case, you need to accurately calculate the amount of the removed edge, because if too much is removed, the piston will touch the open valve. This, in turn, will adversely affect the operation of the motor, and in some cases, even make it unusable, which will cause you to look for a new head.

What Compression Ratio Means and Why It Matters

After revising the cylinder head, it will be necessary to adjust the operation of the gas distribution mechanism so that it correctly distributes the valve opening phases.

Combustion chamber volume measurement

Before you start forcing the engine in the listed ways, you need to know exactly how much of the combustion chamber (the space above the piston when the piston reaches top dead center).

Not every technical documentation of a car indicates such parameters, and the complex structure of the cylinders of some internal combustion engines does not allow you to correctly calculate this volume.

There is one proven method for measuring the volume of this part of a cylinder. The crankshaft turns so that the piston is in the TDC position. The candle is unscrewed and with the help of a volumetric syringe (you can use the largest one - for 20 cubes) engine oil is poured into the candle well.

The amount of oil poured in will just be the volume of the piston space. The volume of one cylinder is calculated very simply - the volume of the internal combustion engine (indicated in the data sheet) must be divided by the number of cylinders. And the compression ratio is calculated using the formula indicated above.

In the additional video, you will learn how you can improve the efficiency of the motor if you improve it qualitatively:

ICE theory: Ibadullaev cycle engine (process)

Disadvantages of increasing the compression ratio:

The compression ratio directly affects the compression in the motor. For more information on compression, see in a separate review... However, before deciding to change the compression ratio, you need to take into account that this will have the following consequences:

  • premature self-ignition of fuel;
  • engine components wear out faster.

How to measure compression pressure

Basic rules for measurement:

  • The engine is warmed up to working temperature;
  • The fuel system is disconnected;
  • the candles are unscrewed (except for the cylinder, which is being checked);
  • the battery is charged;
  • air filter - clean;
  • the transmission is in neutral.

To obtain accurate information about the engine, the compression pressure in the cylinders is measured. Before measuring, the engine is warmed up to determine the clearances between the piston and the cylinder. The compression sensor is a pressure gauge, or rather a compression gauge, is screwed in instead of a spark plug. The engine is then started by the starter with the accelerator pedal depressed (open throttle). The compression pressure is displayed on the arrow of the compression gauge. A compression gauge is a tool for measuring compression pressure.

What Compression Ratio Means and Why It Matters

Compression pressure is the maximum achievable pressure at the end of an engine's compression stroke, when the mixture is not yet ignited. The amount of compression pressure depends on

  • compression ratio;
  • engine speed;
  • degree of filling of cylinders;
  • tightness of the combustion chamber.
What Compression Ratio Means and Why It Matters

All these parameters, except for the tightness of the combustion chamber, are constant and set by the engine design. Therefore, if a measurement shows that one of the cylinders does not reach the value specified by the manufacturer, then this indicates a leak in the combustion chamber. Compression pressure should be the same in all cylinders.

Causes of low compression pressure

  • damaged valve;
  • damaged valve spring;
  • valve seat worn;
  • the piston ring is worn out;
  • worn out engine cylinder;
  • the cylinder head is damaged;
  • damaged cylinder head gasket.

In a working combustion chamber, the maximum difference in compression pressure on individual cylinders is up to 1 bar (0,1 MPa). Compression pressure ranges from 1,0 to 1,2 MPa for gasoline engines and 3,0 to 3,5 MPa for diesel engines.

To prevent premature ignition of the fuel, the compression ratio for positive ignition engines should not exceed 10: 1. Engines equipped with a knock sensor, electronic control unit and other devices can achieve compression ratios up to 14: 1.

For gasoline turbo engines, the compression ratio is 8,5: 1, since part of the compression of the working fluid is carried out in the turbocharger.

Table of main compression ratios and recommended fuels for gasoline internal combustion engines:

Compression ratioPetrol
Until 1092
10,5-1295
from 1298

Thus, the higher the compression ratio, the more octane the fuel needs to be used. Basically, its increase will lead to an increase in engine efficiency and a decrease in fuel consumption.

The optimum compression ratio for a diesel engine is between 18: 1 and 22: 1, depending on the unit. In such engines, the injected fuel is ignited by the heat of the compressed air. And therefore, the compression ratio of diesel engines should be higher than that of gasoline engines. The compression ratio of a diesel engine is limited by the load from the pressure in the engine cylinder.

Compression

Компрессия – это наивысшая степень давления воздуха в двигателе, которое возникает в цилиндре в конце такта сжатия и измеряется в количестве атмосфер. Компрессия всегда выше степени сжатия ДВС. В среднем при степени сжатия около 10, компрессия будет около 12. Это происходит потому что при замере компрессии температура воздушно-топливной смеси растет.

Here's a short video on the compression ratio:

Compression ratio and compression. What is the difference? This is the same or not. Just about complicated

Compression indicates that the engine is operating normally, and the compression ratio determines how much fuel to use for the engine. The higher the compression, the higher the octane number is needed for optimal performance.

Examples of engine defects:

DefectEvidenceCompression, MPaCompression, MPa
No defectsno1,0-1,20,6-0,8
Crack in the piston bridgehigh crankcase pressure, blue exhaust fumes0,6-0,80,3-0,4
Piston burnoutThe same, the cylinder does not work at low speeds0,5-0,50-0,1
Engagement of rings in piston groovesThe same0,2-0,40-0,2
Seizure of the piston and cylinderThe same, uneven operation of the cylinder at idle is likely0,2-0,80,1-0,5
Valve deformationThe cylinder does not work at low speeds0,3-0,70-0,2
Valve burnoutThe same0,1-0,40
Camshaft cam profile defectThe same0,7-0,80,1-0,3
Carbon deposits in the combustion chamber + wear of valve stem seals and ringsHigh oil consumption + blue exhaust fumes1,2-1,50,9-1,2
Wear of the cylinder-piston groupHigh fuel and oil consumption for waste0,2-0,40,6-0,8

The main reasons to check the engine:

Initially, engines were made from such well-known and common materials as cast iron, steel, bronze, aluminum and copper. But in recent years, auto concerns have been striving to achieve more power and less weight for their engines, and this is prompting them to use new materials - ceramic-metal composite, silicon-nickel coatings, polymeric carbons, titanium, as well as various alloys.

Самая тяжелая часть двигателя – блок цилиндров, который исторически всегда делался из чугуна. Основная задача сделать сплавы чугуна с лучшими качествами, не пожертвовав его прочностью, чтобы не пришлось делать гильзы цилиндров из чугуна (так иногда делают на грузовых авто, где такая структура финансово окупается).

Questions and answers:

What happens if you increase the compression ratio? If the engine is gasoline, then detonation will be formed (gasoline with a high octane number is needed). This will increase the efficiency of the motor and its power. In this case, fuel consumption will be less.

What is the compression ratio in a gasoline engine? In most internal combustion engines, the compression ratio is 8-12. But there are motors in which this parameter is 13 or 14. As for diesel engines, it is 14-18 in them.

What does high compression mean? This is when the air and fuel entering the cylinder is compressed in a chamber that is smaller than the standard chamber size of the base version of the engine.

What is Low Compression? This is when the air and fuel entering the cylinder is compressed in a chamber that is larger than the standard chamber size of the base version of the engine.

4 comment

  • Christel

    I'm really enjoying the theme / design of your website.
    Do you ever run into any web browser compatibility problems?
    A couple of my blog audience have complained about my site not working correctly in Explorer but looks great in Firefox.

    Do you have any ideas to help fix this problem?

  • hugechibre78

    I have a huge penis and I like stuffing it in any hole because it's really more than a passion since my childhood with my dad

Add a comment