How Does a Diesel Engine Work and Why is it Efficient?

Most people know diesel engines from way back in the 1970s when they hit the consumer car scene. Every automaker tried to offer at least one diesel-engined passenger car following the gas crunch. To get to the heart of the diesel, you have to go back a lot farther than the ‘70s. The diesel engine was actually invented by a man named Rudolph Diesel, and it wasn’t a recent discovery. It was in 1892 that he sealed the deal by securing the patent for the original diesel engine.

But that’s ancient history. What you really want to know is, “What is a diesel engine?”

Gas Vs. Diesel
To compare these two types of engines you have to know how a gas engine works and how a diesel works. Gas is more common so we can start there. In a modern gasoline engine gas, or fuel, is delivered to each cylinder of the engine by a fuel injector. The injector sprays a fine mist of fuel into each cylinder just above the intake valve. This mixes with air that comes in through the air filter and related air intakes, then flows through the intake valve of each cylinder. Diesel, on the other hand, works on a slightly different version of the same principle. A diesel is an internal combustion engine just like a gasoline engine, but the fuel is delivered in a different manner. In a diesel engine, the fuel is injected directly into the cylinder and mixes with the air there. Since a diesel injector is located within the combustion area of the engine, it’s got to be much tougher than the gasoline version.

The magic of a diesel happens in the cylinders. Where a gas engine needs a spark plug to ignite the fuel and air mixture, a diesel is able to ignite it simply by placing it under great pressure, which creates heat and causes the explosion. As a diesel engine warms up, so does its efficiency rise. It’s an amazing system, and wastes far less energy than the equivalent gasoline setup.

That’s why the MPG rating is so much higher for diesel engines.

Why are diesel engines so noisy?
Back in the ‘70s diesel engines were very straightforward beasts. The compression in a diesel’s cylinder was fast and dirty, which meant it was loud. Everything happened quicly and on top of each other, which wasted very little energy but left us with thousands of sharp sounding little explosions to listen to. The answer to this problem was pre-combustion. Pre-combustion used the heat of the engine to start the combustion process in a small chamber outside of the main combustion chamber, or cylinder, then in a millisecond allow the explosion to jump into the main chamber. This made a quieter engine. Modern diesels don’t have to worry about this thanks to computer aided design and computer controlled engines. They are more efficient than ever.

The Turbo Diesel
Everybody knows that a turbo will make your car faster. But can you make an engine more efficient with a turbo? The quick answer is yes, and doubly so for a diesel engine. Simple engine physics says that the more fuel you can burn in an engine the more power it will make. Dumping gobs of fuel -- diesel or gasoline -- into an engine is easy enough.

But the trick is getting the air to match. Remember you need air and fuel to make the explosion. A turbo physically rams air into the combustion chamber under pressure, which means a lot more air and thus a lot more fuel can go in, making more go-go. But wait, more fuel should mean lower gas mileage, not higher. If you drive a turbocharged car with a lead foot, it’s true, you’ll use more gas even than the same engine without the turbo. But the great thing about turbocharging is that the extra power is available on demand, but only when you demand it. This means that if you drive the car efficiently, you will use less fuel because unlike a car with a huge gas guzzling engine that uses tons of fuel all the time, including the passing lane, your car will sip fuel and only use more in the passing lane.

Thanks turbo!