Efficiency & Power performance are two of the most asked questions when comparing between a diesel and petrol engine. The fundamentals of both engines are similar and the type of fuel burned by either power plant doesn’t change anything in relation to the engine’s general markeup. (e.g. a crankshaft spinning, connecting rods and pistons moving up and down, air being pumped in and exhaust being routed out.) Thus the basic architecture is very much the same. However what goes on in-cylinder in a diesel is vastly different from what you’ll find in its petrol-powered counterpart. The easiest way to explain the difference between petrol and diesel engines is with “air” and “fuel”. In a petrol engine, airflow is everything, you’re throttling air. A diesel mill is the polar opposite. It works on the premise of throttling the amount of fuel being injected – the air simply follows suit.
It starts with “fuel” itself, Diesel which is 15 percent more energy dense than petrol. On the molecular level it comprises of more long-chain hydrocarbons in which carbon and hydrogen are strongly bonded with each other, therefore when energy is supplied in the form of heat, breaking of the carbon/hydrogen bond releasing a much higher energy than petrol.
In addition to burning richer fuel, diesel engines generally have a higher compression ratio because in a diesel engine, only air is being compressed inside the cylinder and it’s known that gas easily compresses than liquid. Air in the diesel engine cylinder generally gets compressed to just 6% of its initial volume, heating up to 400 degrees Celsius which is more than enough to ignite diesel fuel when it is sprayed into the combustion chambers, thus these engines can operate without a spark plug. This is not so in a petrol engine as compression is lower due to the air-fuel mixture inside the cylinder, which requires a spark plug to ignite. This results in a smoother, controlled combustion, but will lesser energy released. This higher compression gives higher heat and simultaneously higher torque.
As we learn that ignition occurs naturally in the diesel engine, therefore, does not require a spark plug, therefore the removal of ignition system not only makes the mechanism simpler but also reduces the risk of improper combustion due to damage in the ignition system, which makes burning of diesel fuel easy and always accessible.
By design, a four-stroke(intake, compression, combustion, exhaust) diesel engine, at the last stroke, the exhaust stroke where spent combustion gasses are forced out of the exhaust valves into the turbine (exhaust) side of the turbocharger. In your average petrol powered engine, no turbocharger exists which means once out of the engine, exhaust gasses immediately head to the tailpipe. With the turbocharger in the diesel engine, it forces fresh air into the engine, while using the exhaust gasses leaving it to drive itself. This is because a turbocharger consists of a turbine (exhaust) wheel sharing a common shaft with a compressor (intake) wheel, exhaust gasses are always required in order to bring air into the engine. One depends on the other. While throttling fuel (sending diesel into the engine), combustion occurs, exhaust gasses leave the engine, spinning the turbine wheel on the way out, which turns the compressor wheel, introducing air into the engine. An endless cycle. The thermal efficiency of the diesel engine is improved by the turbocharger, as it increases the volume of air entering it, which lays the groundwork for combustion of more fuel.
However, Petrol engines are more powerful in the sense that they have a higher acceleration rate due to the higher RPM that the engine can achieve. Given that the torque curve is almost linear, petrol engines achieve peak power at higher RPMs. Petrol engines accelerate faster due to its lesser reciprocating mass and faster energy release.
In conclusion, Diesel engines produce more torque due to the various characteristics as mentioned above, however operating at a lower RPM thus having lower horsepower and acceleration rate. This main difference sets Diesel and Petrol engines apart, with both serving different needs of a wide variety of users.