Petrol engine. A mixture of air and fuel is formed in the intake manifold, after which it follows into the combustion chamber. At the very end of the compression stroke, the final mix of the fuel-air mixture is over and it is uniformly distributed throughout the cylinder. As a result of compression, the mixture is heated to about 500°C, after which it is ignited by the spark plugs. The compression ratio in the petrol engine is 10-12:1. Cars with a petrol engine have a complicated ignition system, which should be properly adjusted. Old automobiles had a relatively primitive carburetor-based fuel system, which was inefficient but cheap. Modern cars don’t use carburetors having replaced them with the direct injection system, in which the combustible mixture is prepared directly in the engine cylinder. Such a system has considerably improved the cost-effectiveness parameters of the engine as well as its price.
Diesel engine. The fuel-air mixture is created much faster in a diesel engine. The air is only compressed in the cylinder of the diesel engine, reaching the temperature of up to 900°C and injected at a pressure of 1500-2000 bar. The compression ratio in the diesel engine is 18:1. Fuel is fed into the combustion chamber separately. Small droplets of diesel fuel quickly evaporate and mix with air. Due to a higher temperature, such kind of mixture ignites spontaneously and doesn’t need an additional spark. As a result, the diesel engine does not have a complicated ignition system which provides a lot of advantages over the petrol engine.
Theoretically, the diesel engine is considered to be more durable due to the rigid block of cylinders, the crankshaft, the cylinder-piston group elements as well as the cylinder head. However, durability directly depends on the diesel fuel quality. From this point of view, the petrol engine is less capricious and more resistant to the low-quality fuel.
Diesel engine, unlike the petrol one, is very sensitive to the low temperatures. Summer diesel fuel thickens at -15°C and ceases to pass through the fuel filter, thus making the car unable to start. However, it’s possible to eliminate this disadvantage using special types of fuel or installing modern heating systems. Additionally, diesel engines warm up for quite a long time, so the passengers feel warmth in the cabin only after 10-15 minutes of fast driving. A diesel engine is water-resistant since electricity is used only to start. That’s why SUVs and crossovers are equipped with diesel engines.
The petrol engine has a small thrust at a low speed and it should be taken into consideration when starting driving. The position of the throttle must be controlled since if it is not opened enough, the engine will shut down. Petrol engine increases its power along with the speed increase providing more convenience during a stop-and-go ride in the urban conditions.
The diesel engine has a high torque at the low speed, which provides a trouble-free start, even when fully loaded. However, the diesel engine has a relatively narrow working range in which the traction power is high (1000-4000 rpm). Therefore, the driver has to constantly monitor the situation choosing the desired gear, so that the engine speed does not go beyond these limits.
Engine efficiency and power
Combustion of the working mixture in a diesel engine is more effective. It is possible due to a high compression rate: 20 units in a diesel engine versus 10 units in a petrol one. Comparing the efficiency of a petrol and diesel engine, it should be noted that the first one is not effective enough and turns only 25-30% of the produced energy into a useful effect. Meanwhile, the efficiency coefficient of a standard diesel engine reaches 40% and the use of turbocharging and intercooling increases its value by up to 50%.
The petrol engine is characterized by a greater power. However, it should be noted that the comparatively small capacity of diesel engines is compensated by a flat traction at any speed, which is beyond the possibilities of petrol engines.
Petrol engines definitely win the price competition. Diesel engines consist of many expensive units and the very assembly process is more complex from a technological point of view. Petrol engines are simpler and therefore cheaper.
Due to the high pressure during the combustion process, diesel engines generate more noise and vibrations. This is a constructive peculiarity of diesel engines. Modern automobiles of a more expensive segment are equipped with a quite good noise insulation. In most cases, being inside the cabin it is impossible to detect what kind of engine is in the car.
The ignition of the liquid in a petrol engine is made using the spark, while in diesel engine it is spontaneous with a compression ratio of 1:25. In the case of the petrol engine, this figure is 1:12. Thus, fuel combustion is more effective in diesel engines and this fact directly influences the level of engine’s impact on the environment.
Since the diesel engine has higher efficiency coefficient and fuel combustion rate, it produces less harmful emissions than the petrol one. Leading world auto manufacturers significantly reduced the level of harmful substances in the exhaust gases that are thrown into the atmosphere, thus making diesel engines more environmentally friendly. Even despite recent scandals with automakers, diesel engines still remain the most ecologically among internal combustion engines. In this regard, only hybrids and electric cars show better results.
Diesel engines emit the smallest particles of soot which are considered particularly harmful to humans. In addition to soot particles, diesel engines emit more nitrogen dioxide compared to petrol engines. A prolonged contact with this substance worsens lung function and provokes allergy. Modern technologies offer an acceptable solution introducing a new Euro 6 environmental standard, which entered into force in September 2015. It provides a reduction of nitrogen dioxide emissions by half for diesel vehicles manufactured after September 2015.
Due to the solid design of the cylinder block, a diesel engine is considered to be more durable. Owners of automobiles with diesel engines have to change filters and oils and check the compression in the cylinders more frequently. Diesel units are characterized by a complex design, so service specialists may not be able to eliminate any breakdown. As a rule, the repair of a diesel engine is more expensive since the spare parts for it cost more. The reason is that diesel engine is all about an ultra-high fuel pressure. At such pressure, heavy fuel equipment can’t be simple and cheap even at the level of the materials it is made of.
The most common problems with diesel engine arise from the use of a low-quality fuel. These engines are very sensitive to the quality of fuel. Excess of sulfur content from 0.2% to 0.5% reduces the engines resource by 25%!
A diesel engine requires more capital investments, but only if talking about the short term. If a car is purchased for a long time (from 5 years) aimed at passing at least 20 000 kilometers per year, then it’s possible to save money due to lower fuel consumption.
The fuel system is the most expensive one in a diesel engine. It is especially true when it comes to a high-pressure fuel pump, whose parts are produced with a very high level of precision and the interior parts are lubricated with diesel fuel. Therefore, the work of the diesel engine in terms of fuel absence inside the fuel pump inevitably leads to its breakdown.
The vehicle with a petrol engine is less sensitive to the fuel quality. The speed and power of the engine are very high and the spare parts for repair are more affordable.
Modern petrol engines are also very complex technical units and almost equaled the diesel engines regarding the cost of service. Still, they are preferable when it comes to periodic maintenance works. Usually, the mileage between the compulsory maintenance tests in modern diesel engines is 10 000 km, compared to 15,000 km in petrol ones.
Automobiles with diesel engines are a bit heavier. However, this fact is not critically important and does not exert a substantial impact on the driving performance of the car.
The difference between diesel and petrol fuel is as follows: diesel fuel evaporates more slowly, which reduces the likelihood of a fire. In addition, the ignition system is not used in diesel engines.
Diesel engine demonstrates a higher compression of fuel than a petrol one that is caused by its constructive features. Because of this fuel consumption in diesel engines is significantly lower. Also, a diesel engine has a higher efficiency coefficient and at maximal revs, it still remains equally economical. Fuel consumption in an urban mode for a diesel engine is 5.8 liters/100 km on average, for a petrol one – 9.2 liters/100 km. Accordingly, the automobile on diesel fuel consumes 0.058 liters per 1 km, while on petrol fuel – 0.092 liters.
– higher efficiency coefficient
– high durability
– higher ecological compatibility
– an engine is not afraid of water due to the lack of an ignition system
– high power at any speed
– consumes less fuel – fuel system is extremely sensitive to low-quality fuel
– maintenance works should be carried out more often
– poor resistance to low temperature
– increased noise and vibration
– complexity and a high cost of maintenance
– resistance to low temperature
– relative cheapness and simplicity of maintenance
– low noise level
– easily tolerates work at a very high speed
– tolerates the fuel quality – increased fuel consumption
– availability of spark plugs for ignition
– increased explosiveness
– lower efficiency coefficient
– negative impact on the environment