Why is it difficult to design a high-speed diesel engine?

The dashboard of a diesel car is very different from that of a gasoline car. The biggest difference is in the tachometer: the tachometer of a gasoline car can display a maximum value of 7000-8000, and the tachometer of a cylinder motorcycle can display a maximum value of 7000-8000. The speed can reach 15,000 rpm, while the maximum speed of the tachometer of a diesel vehicle is usually around 4,000 rpm.

The maximum speed of the truck tachometer is only 2,800 rpm, and the most economical speed range is usually 1,500 rpm. It is not recommended to be used in the high-speed range. Many people wonder: Why can the speed of a gasoline engine be so high? And the diesel engine speed is so low? What would happen if a diesel engine was also designed for high speed? Is the diesel engine speed very low?

In fact, this issue should be viewed from two perspectives. Commercial vehicles and multi-purpose vehicles use large-displacement, heavy-duty diesel engines, and passenger cars use light-duty diesel engines, such as Volkswagen TDI models. PSA has AHDI All models can reach 5000rpm, and the fuel cut-off speed is about 5500rpm. In fact, most turbocharged gasoline engines have a fuel cut-off speed of about 6,000rpm, so in the field of passenger cars, the speed of diesel engines is not low.

In the field of passenger cars, the speeds of diesel engines and gasoline engines are already very close. However, in the case of large-displacement, heavy-duty diesel engines, due to the large mass of pistons, cranks, crankshafts and other components, the machine is running During the process, the inertial moment generated is too large, so it is impossible to rotate fast, and the body cannot bear it if it rotates too fast. This is the problem that large-displacement, high-power internal combustion engines, and even gasoline engines cannot rotate quickly, which is needed and applied. the difference.

Because the basic condition for the normal operation of the engine is that the power stroke has enough time for the mixture to burn to produce energy. For example, when the engine speed is 8000rpm, the power stroke of a single cylinder only takes 0.225 seconds. For a gasoline engine, the time It's enough, but it's a bit weak for a diesel engine. Since gasoline easily catches fire, when a gasoline engine is running, it starts mixing gasoline and air during the intake stroke, and then mixes further during the compression stroke.

As a result, the mixture can be burned immediately and quickly, with higher internal energy and earlier explosion, pushing the piston to achieve high fuel consumption in the diesel engine, but at the same time inevitably causing more severe vibrations in the piston and cylinder of the engine , not rigidly connected by guide rails or other methods, the piston enters the circular cylinder, and there is a gap between the cylinder walls. Due to the high intensity of vibration during operation, the degree of "collision wear" of the piston and cylinder wall of the diesel engine increases, and the gasoline engine with low intensity vibration will have an opportunity smaller.

So the diesel engine cannot be designed as a high-speed engine, otherwise the wear will be very large. As we all know, the displacement of a diesel engine is 30-40% larger than that of a gasoline engine. Even for the same model, due to the long stroke of the diesel engine and the heavy piston, Unable to rotate at high speed, there is no choice but to rely on recharging. The supercharger introduces a large amount of air, increases oxygen concentration, reduces black smoke, burns more fuel, and generates torque.

As the amount of inhaled gas increases, the heat absorbed by it also increases, the combustion temperature decreases, and NOx also decreases. The diesel engine turbocharger is not only related to performance, but also an indispensable part to prevent exhaust emissions. Therefore, as the diesel engine speed increases, the power stroke time shortens (the time occupied by the four strokes shortens), resulting in extremely incomplete combustion of the diesel engine and extremely uneven mixing. If the combustion time is shortened, the mixing will be aggravated The problem of unevenness ultimately leads to compression ignition failure, that is, misfire.

So the high viscosity and poor volatility of diesel are the fundamental reasons that affect the speed. Of course, high-pressure rail technology has improved many problems, but compared with gasoline engines that can run tens of thousands or even tens of thousands of revolutions, Diesels have a hard time doing that.