Why Diesel Engine Have Low rpm? 

KPV510 powerful 12 cylinder diesel engine has the characteristics of high compression ratio and low speed. So why is the speed of diesel engine relatively low?

1. The Relationship between Horsepower, Torque and Speed

Horsepower, torque and speed are essentially three interrelated parameters. The relationship between these three parameters can be seen from the following relation expression.

Torque * speed * n = power, where n is a constant.

Through the above formula, the relationship among torque, speed and power can be analyzed. Power is to describe how much work the engine does. If the power is higher, it proves that the stronger the power capacity of the engine in unit time is, the greater the kinetic energy that can be provided to the vehicle will be. Naturally, the car runs faster.

Torque is to describe the amount of force that an engine crankshaft cranks. The greater the torque is, then the more traction is provided to the vehicle. According to Newton's law, it is easy to conclude that the greater the engine torque is, then the faster the vehicle will accelerate, and the stronger the towing capacity will be. The unit of speed is several revolutions per minute, which means several revolutions of the crankshaft per minute. Therefore, in the case of constant gear, the engine speed increases, and the vehicle speed also increases.

2. The Difference between Gasoline Engine and Diesel Engine in Combustion

After the mixture gas in the gasoline engine is ignited, it burns instantaneously and produces energy. So the cycle can be repeated many times in a unit time, and high power can be output with high speed. Therefore, with small volume and light weight, high performance and faster response speed can be obtained. The wide speed range can also bring better control feeling. However, the compression ratio of gasoline engine is usually only half of that of diesel engine. The temperature and pressure in cylinder during power stroke is much lower than that of diesel engine, so the thermal efficiency is relatively low, which is commonly known as‘high fuel consumption’.

After the fuel is injected into the KPV510 12 cylinder diesel engine, the combustion takes a certain time, so it is suitable for the full combustion of the fuel at a lower speed to bring large torque. In order to resist the high pressure and large torque in the cylinder, the cylinder, the connecting rods of piston of the diesel engine and other parts are stronger than those of the gasoline engine, so they are more bulky than the gasoline engine. But it is also because of the high compression ratio and low speed characteristics of the powerful 12 cylinder diesel engine, the heat can be better converted into kinetic energy, so the diesel engine has better thermal efficiency, that is, better fuel consumption performance. This is why cars and racing cars usually use gasoline engines, while buses, trucks and other large vehicles use diesel engines.

3. Speed and Torque Difference between Gasoline Engine and Diesel Engine

The real difference between a gasoline engine and a high speed 12 cylinder diesel engine is in its speed and torque. The torque curve of gasoline engine is relatively steep, and the speed range corresponding to the maximum torque is very small. The torque of diesel engine is not only larger than that of gasoline engine with the same displacement, but also the speed range corresponding to high torque is wider than that of gasoline engine. That is to say, the low-speed torque of diesel engine is much larger than that of vehicle with the same displacement, and it is much easier to start under heavy load, which is the reason why diesel engine is more powerful. Because of this, today's convoy trucks and construction machinery all use diesel engines. Gasoline engines do not work in these areas.

Power (or horsepower) is the product of torque and speed. Although the torque of high quality 12 cylinder diesel engine is large, the high torque appears at low speed, but the torque comes down when the speed is high. The torque of gasoline engine is low when the speed is low, and the torque is high when the speed is high. The result of multiplying two large numbers must be greater. This is the reason why the power of diesel engine should be small with the same displacement, because both have their own characteristics.