Gasoline engine 'steal' from a diesel engine

Those who know a little about the engine understand that there is a big difference between a gasoline engine and a diesel engine. The most obvious difference is of course that the gasoline engine burns gasoline and the diesel engine burns diesel oil. For the combustion method, the two are also significantly different from the gasoline engine, the gasoline engine is ignited, and the diesel engine is ignited. In comparison, the compression of gasoline engines is relatively low, generally between 9-11, while the compression ratio of diesel engines is high, generally 15-22. The combustion efficiency of a diesel engine is also higher than that of a gasoline engine, which is generally 34%--45%, while that of a gasoline engine is only about 30%. The main emissions of gasoline engines are HC and CO, while the main emissions of diesel engines are NOx and particulate matter (PM). Therefore, the post-treatment devices are also different. The gasoline engine is mainly a three-way catalyst, while the diesel engine mainly has a DPF (particle trapping). ) and SCR (Selective Catalytic Reduction).

The diesel engine has large power and high thermal efficiency, and the gasoline engine has become jealous. Therefore, many technologies have been “stolen” from the diesel engine. After the engineers' adjustment and optimization, the performance of the gasoline engine is continuously improved and the thermal efficiency is improved, thereby achieving the purpose of energy saving and emission reduction. .

NO.1 direct injection in cylinder

Since the diesel engine fuel is diesel, its viscosity is greater than that of gasoline and it is not easy to evaporate, but its auto-ignition temperature is lower than that of gasoline. When the diesel engine is working, the air entering the cylinder is air. When the air in the cylinder is compressed to the end point, the gas is in a state of high temperature and high pressure (T=500-700°C, P=40-50 bar). When the piston approaches TDC, The fuel injection nozzle of the oil supply system injects fuel into the combustion chamber of the cylinder in extremely short time with extremely high pressure (generally 2000bar). Diesel oil forms fine oil particles, mixes with high temperature and high pressure air, combustible mixture burns itself, and violently The explosion produces explosive power and pushes the piston to do its work.

The gasoline has low viscosity and rapid evaporation. The injection system of the port injection engine injects gasoline into the intake passage and mixes with air to form a combustible mixture. After being compressed to a certain temperature and pressure, it is ignited with a spark plug. Gas expansion does work.

In-cylinder direct injection (GDI) technology is a gasoline engine stolen from a diesel engine. The injection system of a direct-injection engine directly injects gasoline into a combustion chamber to form a gas-oil mixture. In this way, the direct injection engine of the cylinder can accurately know the amount of air that enters the cylinder to burn according to the timing of the opening and closing of the intake valve, so as to determine the amount of fuel to be ejected according to the intake air amount. The amount of air intake can be determined by the load of the entire vehicle. At idle or low load, the intake valve opening is small and the fuel injection amount is small. When the load is high, the intake valve is fully open, the intake air volume is large, and the fuel injection amount is also large, ensuring that the engine power can meet the demand. In this way, both the power of the engine and the goal of energy saving and environmental protection can be achieved. But it needs to be emphasized that even a gasoline engine with direct injection in the cylinder will still be a mixture of oil and gas. It will only change the position of the fuel injection from the intake to the combustion chamber. The diesel engine compresses the air. This is the cylinder Direct injection gasoline engines and diesel engines are significantly different. Moreover, although the direct-injection gasoline engine in the cylinder is also a high-pressure injection (about 20bar), it is less than the high pressure of a diesel engine.

NO.2 Turbocharged

Turbocharging technology was first adopted on marine diesel engines, and was gradually adopted on military aircraft and warships. Caterpillar was one of the first companies to promote turbocharger technology to passenger car engines in the early development of automotive turbocharged power. Until the 1970s, turbocharger technology ushered in a period of great development because of the oil crisis. The oil crisis that broke out in 1973 led directly to the skyrocketing prices of gasoline and diesel worldwide. At this time, diesel vehicles equipped with turbochargers in the commercial vehicle market began to be hot. Subsequently, the turbocharged engine began to shine on the track. In 1968, the STP-Paxton car with a turbocharged engine won the Indy500 event. Since the turbocharger was first carried on a diesel engine and was then carried on a gasoline engine, it could be considered that the gasoline engine was stolen from the diesel engine.

Turbocharging principle

Turbocharging uses the inertial impulse of the exhaust gas emitted by the engine to propel the turbine in the turbine chamber. The turbine drives the coaxial impeller. The impeller compresses and sends the air from the air filter duct to pressurize and then enter the cylinder. When the engine speed increases, the exhaust gas velocity and the turbine rotation speed also increase at the same time. The impeller compresses more air into the cylinder. The increase in air pressure and density can make more fuel fully burned, and accordingly increase the amount of fuel. By adjusting the engine speed, you can increase the output power of the engine.

The high-tech artifacts publicly promoted—the engine in the TSI DSG's gold combination—use a combination of in-cylinder direct injection and turbocharger technology. In fact, both are stolen from the diesel engine, but in Europe. In passenger cars, the proportion of diesel vehicles is no lower than that of gasoline vehicles, and there is a wealth of people who have studied and manufactured diesel engines. It is also reasonable to transplant the advantages of diesel engines to gasoline engines.

NO.3 high compression ratio

The ratio of the total cylinder volume of the internal combustion engine to the volume of the combustion chamber is an important structural parameter of the internal combustion engine. When the piston is at the bottom dead center, the cylinder has the maximum volume, which is denoted by Va. When the piston is at top dead center, the volume in the cylinder is called the combustion chamber volume, which is represented by Vc. The compression ratio ε=Va/Vc of the internal combustion engine indicates the degree to which the gas in the cylinder is compressed when the piston is moved from the bottom dead center to the top dead center. The gasoline engine is generally 9-11, and the diesel engine is 15-22. Therefore, the compression ratio of the diesel engine is much higher than that of the gasoline engine.

The compression ratio has many effects on the performance of the internal combustion engine. The higher the compression ratio, the higher the thermal efficiency, but as the compression ratio increases, the thermal efficiency increases less and less. The increase of the compression ratio increases the compression pressure and the maximum combustion pressure, so that the mechanical efficiency of the internal combustion engine decreases. But overall, increasing the compression ratio can increase the efficiency of the engine. This is also a key reason why the efficiency of diesel engines is greater than that of gasoline engines.

Why, then, does the gasoline engine not increase the compression ratio? This is because gasoline engines are prone to knocking if the compression ratio is too high. If the degree of knocking is severe when knocking occurs, it is extremely harmful to the engine. At this time, the combustion reaction in the cylinder is extremely violent, and the pressure changes abnormally. The shock wave generated thereby will cause the gas in the combustion chamber to vibrate, and even hear the metal knocking noise in the cylinder of the engine. In severe cases, the engine will be damaged. Break down the cylinder and cause irreversible damage.

Therefore, the limiting factor for increasing the compression ratio is knocking. The average gasoline engine compression ratio is around 11 and it can be considered very high when it reaches 12. Mazda externally declares that the engine of Chuangchi Blue Sky Technology can achieve a compression ratio of 13:1, but This "compression ratio" has a lot of articles. This engine can be seen as an Atkinson cycle (or Miller cycle). From the definition of the compression ratio we can understand the cylinder volume ratio at the top and bottom dead center of the piston. However, the main aim of the Miller cycle was to pursue a "expansion ratio greater than the compression ratio," so the intake valve did not close during the beginning of the compression stroke, but secretly released the air just inhaled, so that it appeared that the actual compression The stroke is shortened and the actual compression ratio is far from 13:1. This explains why engines with such high compression ratios can not use high-grade fuel. In fact, in a broad sense, these engines are collectively referred to as “Atkinson-Miller Cycle Engines” and the principle is the same. The purpose is to improve thermal efficiency.

From this we can see that the high compression ratio is a magic weapon for diesel engines. The gasoline engine has been working hard for so many years, and it has only risen a little bit. It is far less than the compression ratio of diesel engines. It can be described as “stolen” and “stolen” without going, but the car Engineers are still making unremitting efforts to increase the compression ratio of gasoline engines without any strong knocking through various means, thereby improving the thermal efficiency of gasoline engines.

NO.4 compression ignition

"Gasoline is ignited, diesel is ignited." This is a common sense about the engine, but this common sense is gradually being broken. Gasoline engine is "stolen" the magic weapon of diesel engine at the bottom of the box - compression ignition.

Homogeneous Charge Compression Ignition (HCCI) is a new combustion mode based on a reciprocating gasoline engine. In simple terms, it is a compression ignition method for gasoline engines. It combines the traditional gasoline engine homogenous mixture. The advantages of high combustion efficiency with diesel engines have great potential for efficient, low-emission combustion. Like the traditional gasoline engine, the HCCI engine injects a very uniform ratio of air and fuel into the cylinder. Traditional gasoline engines fire through spark plugs, igniting the air and fuel mixture to produce energy. However, the HCCI engine is different. Its ignition process is similar to that of a diesel engine. It is burned by raising the temperature of the piston to a certain extent by compressing the mixture.

To realize HCCI, the engine must be modified. Variable compression ratio, EGR and VVT are the means to achieve HCCI. However, achieving HCCI has great limitations. Only HCCI can achieve the best performance under medium and low load speed conditions. The combustion efficiency and emissions. In the start-up phase and high-speed and high-load operating conditions, traditional ignition methods must also be used. Currently accepted by the mainstream is the use of hybrid models with electric motors or HCCI/SI dual-mode engines.

In terms of theoretical research, Professor Li Liguang of Tongji University has achieved HCCI/SI switching by changing the EGR rate and VVT. In actual application research and development, Mercedes-Benz and GM are at the forefront, and their designed HCCI engine can save about 15% of the fuel compared with traditional engines. It is believed that as technological difficulties are gradually overcome, HCCI technology will rapidly spread to the public as a new energy-saving and efficiency-enhancing technology, making a contribution to the blue sky of the earth.