The principle of operation of diesel power units is based on the ignition of the fuel mixture in the cylinders under high pressure. Unlike a gasoline engine, a diesel engine does not have a spark supply system using electrode tips on candles: an oxygen-enriched mixture is fed into the combustion chamber, where high temperature is formed during compression. Instead of forming an electric spark, additional glow plugs provide additional pre-heating.
The main parameter for diagnosing the condition of a diesel engine is compression, on which the power and stable operation of the mechanisms of the cylinder-piston group depend.
What is diesel engine compression
Compression is an indicator of the pressure in the cylinder that is created by the piston at the maximum highest (dead) point of the beat. In the technical literature it is customary to use the abbreviation TDC (highest dead center). The value is measured in kg / cm² and depends on two main factors:
- Loss of tightness of the combustion chamber: air leaks, the pressure in the system drops. Deterioration of valves, cylinder-piston units, or incorrect adjustment of the gas distribution mechanism leads to a decrease in compression.
- The condition of the crankshaft and its rotation speed: the loss of stable revolutions leads to heat loss during compression of the air by the piston. The degree of compression becomes lower than normal, which directly affects the energy that is generated as a result of the formation of working gases during the combustion of the fuel mixture in the cylinders.
The rapid wear of the CPG mechanisms (cylinder-piston group) depends on the quality of the lubricants, the intensity of the loads during operation, the number of cold starts at sub-zero air temperatures and hours of operation at maximum speeds. Incorrect adjustment of the valve system contributes to the formation of soot on the walls of the combustion chamber, cylinder liners, coking of the piston and oil scraper rings. As a result of insufficient pressure, the fuel mixture does not burn out completely, mixes up with oil residues, leaving burning products, soot on the CPG units. The first sign of deterioration in this case is smoky exhaust and a drop in engine power.
During the start and operation of the starter, the crankshaft in the diesel unit rotates at a speed of 200-300 rpm: the higher the torque, the higher the compression. If the pressure in the combustion chamber is below 16 atmospheres, it will become impossible to start a diesel engine in the usual way with a key. Heat losses cause a decrease in the degree of compression and a violation of the adjustment of the valve timing.
Compression rate in diesel engine cylinders
Depending on the power of the power unit, the type of construction of its combustion chamber and injection system, the compression rate may be different. It must be sought in the technical description of each particular brand. Below are the standard indicators of the ratio of starting temperature and compression, at which it is possible to start a diesel engine of any type:
- Less than 18 kg / cm². engine start is impossible even with forced heating of the system;
- 22-24 kg / cm². start-up is possible only on a hot unit, after a temperature drop below 0 ° C, failure to start from the starter is guaranteed;
- 24-26 kg / cm². starts at an air temperature of 10 ° C;
- 26-28 kg / cm². maximum start temperature. 15 ° C;
- 28-32 kg / cm². cold start up to. 25 ° С is possible;
- 32-36 kg / cm². cold start up to. 30 ° С is possible;
- 36-40 kg / cm². cold start up to. 40 ° C is possible.
These compression standards are applied only when all other systems responsible for the operation of the engine are in good working order: fuel supply (high pressure fuel pump), 100% battery charge, serviceable timing assemblies. Modern types of diesel engines with a separate combustion chamber and the Common Rail fuel mixture injection system have better results: on average, for each compression indicator there will be five temperature values. The error also increases with the number of cylinders in the unit and the type of layout: V-shaped for 6 and 8 cylinders have a greater advantage over conventional in-line four-cylinder.
How to measure cylinder pressure
Compression readings are performed using a special device. a compressometer. The compressometer works according to the principle of a manometer: an adapter with a nozzle or glow plug design is designed to be installed in the cylinder head and has a shut-off valve that prevents pressure loss during measurements. Simple devices are equipped with a conventional scale with an arrow, more accurate professional devices. electronic digital, or synchronized with a computer.
Before measurement, the diesel engine is heated at maximum speed to the operating oil temperature, the operation of the starter and the state of charge of the battery are checked at 100%. Each cylinder is measured separately.
- Unscrew the glow plug or nozzle from the head of the unit (depending on the type of diesel unit);
- the adapter fitting of the compressor is inserted into the place of the candle (nozzle), making sure the tightness of the standard connection;
- turn on the ignition without starting the engine before the starter turns. the initial crankshaft revolution (rotation speed must be at least 200 rpm);
- take the measurement result (at the top dead center point of the cylinder stroke);
- measure compression in the remaining cylinders in a similar manner;
- compare the compression ratios of all cylinders;
- then about 50 ml of oil is injected into each cylinder;
- crank the cranks as often as the starter with the injectors (candles) removed;
- set the nozzles in place and conduct new measurements on each cylinder again;
- check the difference in readings with the first measurement.
When reconciling pay attention to the discrepancy in the compression ratio of each cylinder: the minimum error should not exceed 1 kgf / cm². Equal values of measurements indicate the same wear of the nodes and mechanisms of the cylinder-piston group.
If the results of the second measurement exceed the permissible error rate, then the system is leaking. Possible causes of violations in this case: violation of valve clearances (defects), piston ring wear, leakage in the cylinder head housing (burnout), piston cup breakage. If insufficient engine compression is detected, the cause of the malfunction is diagnosed and established.
Why compression may go down
The pressure drop in the cylinders is accompanied by the following characteristic features:
- failure to start the engine at low temperature or poor response of the ignition system;
- during idling, “triple” occurs;
- power drop with unstable turns;
- increased fuel consumption;
- excessive oil intake;
- extraneous impurities of exhaust gases (smoky exhaust);
- loud noise, vibration during throttle opening.
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Reduced compression occurs due to air leaks in the gas distribution system, cylinder block or wear of elements and components of the cylinder-piston group. The timing may not work correctly due to the formation of soot on the valve cups, damage to the drive mechanisms (springs, saddles). To identify a specific cause, the valve system is diagnosed. valve clearances are measured.
In case of wear of the CPG parts (cylinder-piston group), the cylinder head is removed to find a worn piston cup, gaskets, piston rings or the working surface of the cylinders. Burnouts, cracks on the body of the head of the block can be installed visually.
If reduced compression is detected, a full check of all systems is performed. Modern diagnostic methods using a special “G ‑ Scan” scanner make it possible to determine the cause of a breakdown accurate to a specific unit based on an analysis of the fuel mixture data, compression values and various sensor readings. The scanner takes into account the entries in the ECM (electronic engine control system) journal, where deviations in the operation of the systems are detailed, all malfunctions in the timing mechanisms are taken into account according to the error codes.
On-board computer diagnostics will provide an accurate definition of a violation that can be skipped during a visual inspection. For example, in the process of insufficient supply of the fuel mixture into the cylinder due to a violation of the clearance of any valve, the block controller, based on the readings of the oxygen sensor, regulates the additional injection to a specific cylinder, giving a command to the actuator. During a routine check with a measurement of the gaps of all valves, it is impossible to establish the cause of soot in a particular area.
The reason for the pressure drop may be due to a slight air leak, when a simple adjustment or replacement of the gasket is sufficient to fix it. In case of serious wear of the important mechanisms of the CPG, an overhaul of the engine will be required.
How to start the engine with insufficient compression
At low compression ratios, the diesel unit does not start with the usual cranking of the starter. If starting at minus temperature on a cold engine, then the force for normal speed of rotation of the crankshaft requires more, since the oil becomes more viscous in frost. To heat the cardan, a conventional blowtorch is used, which is installed at the bottom and the engine systems are heated with hot air.
It is also possible to artificially increase compression for a single cold start by creating an oil film in the cylinders. To do this, turn out the glow plugs, or nozzles and pour motor oil with a syringe directly into the combustion chamber, with a volume of not more than 50 ml. Repeat the whole procedure on each cylinder separately. After cranking the crankshaft so that a uniform oil film forms on the walls, which will ensure the creation of tightness during start-up. Using this method, it is necessary to trace the distribution of oil in the cylinder, before starting the engine, the piston must make several strokes with a starter to avoid hydrocline.
Author: Maxim Markov
The date: 2017-07-31
Features of the working process of a diesel engine are such that the normal course of the working process is possible with a combination of several factors. One of the most important indicators is compression. If this indicator deviates, a difficult start or unstable motor operation is possible. Therefore, it is very important to know what compression should be in a diesel engine.
- What it is?
- What should be the pressure in the cylinders
- Reasons for Pressure Drop
- How to determine a possible reduction in compression
- Measurement technique
- What to do when reducing compression
What it is?
Compression in a diesel engine is the maximum pressure created by the piston after closing the intake valve and raising it to the top dead center. The duty cycle of a diesel engine is closely connected precisely with the creation of high pressure in the cylinder, which provides ignition of the fuel from air compression in the combustion chamber when a sufficiently high temperature (about 300 degrees) is reached. Therefore, we can say that compression in the cylinders of such engines is the key to its stable operation.
Unlike gasoline engines, in which the ignition of the air-fuel mixture occurs due to an electric spark, in a diesel engine ignition occurs only due to compression, so the pressure there reaches much higher values. In gas turbine engines, air is first compressed, and then, in the combustion chamber, mixed with fuel. Another indicator that indirectly informs about internal work processes is the degree of compression. This is the ratio of the cylinder volume when the piston is located at the lowest point to the volume of the combustion chamber at the top (with the piston maximally raised). The resulting figure indicates how many times the fuel mixture is compressed in the cylinder at the time of ignition (end of the compression stroke). This value is dimensionless, since it characterizes the ratio of values. Compression is measured in pressure units. in bars or kg / cm2.
What should be the pressure in the cylinders
For gasoline engines, the normal compression ratio and compression are 9–11 and 12–13 kg / cm2, respectively; for diesel engines, the norm is 20 or more and 20–32 kg / cm2. As you can see, the indicators vary greatly, and this is the difference in their workflow.
Low ambient temperatures can be a serious obstacle. Different pressure standards can provide start at different temperatures:
- 28 kg / cm2 and below. engine launch is possible at a temperature not lower than minus 15 degrees.
- 20-30kg / cm2. temperature up to minus 20 degrees.
- 32 kg / cm2. minus 20-25 degrees.
- 36 kg / cm2. up to minus 30 degrees.
It should be noted that such temperature ranges on a diesel engine are achievable only with a fully operational power unit and auxiliary equipment.
Reasons for Pressure Drop
The compression ratio is incorporated in a constructive manner and can only be changed by installing other pistons or the cylinder head with other geometric parameters. Compression of a diesel engine is a variable value. Over time, critical parts wear out. the gaps of the compression and oil scraper rings on the pistons increase, the valves, the cylinder block gasket, as a result of which the tightness is broken and the necessary pressure is no longer created. On a gasoline engine, this is fraught with unstable transient operation and a drop in power, and a diesel engine may simply not start.
How to determine a possible reduction in compression
There are several signs by which you can find out about its decline.
- Difficulties at startup (both cold and hot).
- Failure of one of the cylinders, misfire
- Sudden change in speed, their unevenness.
- Decrease in power, failures at work.
- Overheating of the power unit, increased pressure in the cooling system.
- Blue smoke from the exhaust pipe at startup.
- Increased noise during operation, vibration at any speed, especially the minimum.
- Oil drips on the surface of the power unit.
- A significant increase in fuel consumption.
To measure the compression of a diesel engine, certain special equipment is required. When carrying out measurements, a compressorometer is required. a variant of the pressure gauge adapted to measure pressure inside the cylinders.
How to measure compression? The work consists of the following steps:
- First you need to remove the glow plug or nozzle on one of the cylinders.
- Instead, the pressure gauge fitting is screwed in.
- The crankshaft is cranked with a starter, at which time pressure indicators are recorded. The starter must provide at least 200 rpm.
- The pressure gauge is disconnected and the check is carried out similarly in all other cylinders. Normally, the measurement results should be approximately the same in all cylinders.
- Spark plugs or nozzles are again twisted and 50 ml of engine oil are injected into each cylinder through the hole.
- The engine is scrolled by the starter with the candles or nozzles removed. After that, the compression in all cylinders is checked again.
If the indicator has changed in a larger direction, then there is wear on the parts of the connecting rod and piston group in the cylinders of the diesel engine.
If the compression indicators are the same, you should adjust the gas distribution mechanism (thermal valve clearances).
After making the necessary adjustments, it is necessary to once again check the compression and eliminate the malfunctions of the auxiliary equipment.
What to do when reducing compression
How to increase compression? Low rates are usually the result of wear and tear with its high mileage. Just like a gasoline engine, such a diesel engine is often subject to major repairs, namely: replacing the connecting rod and piston group, grinding the valves, replacing the gaskets of the cylinder head. Even if checking the compression of a diesel engine shows its decrease in only one of them or in several.
Nevertheless, it is possible to postpone an expensive repair a little using special means to increase compression. This is, first of all, a variety of additives that are added when replacing oil for diesel engines. They modify its initial composition and increase the viscosity characteristics, thus, it is possible to minimize pressure leakage through the compression rings and increase the compression. Also, to increase the pressure and reduce leaks, grinding the valves is carried out.
Diesel repair is much more expensive due to design features and high technical requirements for diagnostics and special equipment. Therefore, it is important to regularly measure the compression of a diesel engine in order to prolong its performance using timely preventive measures.