American Dream DIY Auto Repair

Introduction to Diesel Motors

When some people hear the words, “diesel” sometimes they just think of nothing but a green pump at the gas station and have absolutely no idea what the difference is from a gas motor. I mean, who could blame them? They both do the same job of turning the wheels right? Yes and no; even if you put the biggest gas motor money could buy into a Peterbilt hauling 50,000 lbs down the road, it might turn the wheels but you wouldn’t see past 15 maybe 20 mph. They key lies in the explosive power within the cylinders that a diesel motor contains. There is no spark, or igniter in the cylinders. Instead, the huge amount of compression inside the cylinders is what ignites the fuel-air mixture. Let’s take a look at the difference between a gas and diesel motor in terms of parts.

The key difference in these two photos is the absence of a spark plug in the diesel motor. That is the basic concept that sets these two motors apart; yet it has so much to do with the performance of each motor.

Concepts

The fuel and air mixture enters the diesel motor much in the same process as the gas motor, however there is usually a turbo charger which consumes a considerable amount more of air than naturally aspirated motors. Along with, a fuel injection pump is usually in the mix as well,providing higher injector tip pressures to completely atomize the fuel. The mixture in the cylinder ignites when the pressure has heated it to approximately 1,400 degrees Fahrenheit. The combustion tends to happen at a more constant pressure instead of a constant volume. Diesel engine combustion also tends to occur at constant pressure rather than at constant volume, as in a gasoline engine. This means that in the diesel engine the combustion pressure continues to rise steadily as the piston retreats and the cylinder volume increases, whereas in the gas engine, the combustion process is so rapid that there is very little movement of the piston while it occurs and, thus, very little increase in cylinder volume. The expansion of the flame at a constant pressure is responsible for the diesel’s reputation for extreme torque.

Another large difference between the motors is the comparison of horsepower to torque. A diesel engine is limited in terms of horsepower because of the equation that derives the term “horsepower” which is work over time. Work being the power of the motor and time being the RPM. Since the burn rate of diesel is a lot slower than that of gasoline, it is harder for the diesel motor to achieve higher RPMs thus lessening the amount of total horsepower being produced by the motor.

Downsides of Diesel

One subject that the gasoline motor has over the diesel is the amount of power to weight ratio. Since the compression within the diesel motor is so great, the walls of the cylinder and the motor in general has to be very robust and strong. Because of this, the weight to power ratio of the diesel motor can be 1.5 to 3 times that of the gasoline motor. Another reason for that poor ratio is the power stroke or cylinder event time of the diesel can be very slow due to the slow burning and slow mixing time of the fuel compared to gasoline. In simpler terms, the bang can’t keep up with the rotation of the motor for the bang to have any effect on horsepower.

Turbo chargers, fuel injection types, and a more efficient burn will all be discussed further in the Intermediate learning section of these articles. These subjects can become very complex when really boiled down which is why we’re choosing not to enclose those in this post.