One of the most common questions when it comes to turbochargers is how they differ when used in petrol driven cars as opposed to diesel. The basic set up of each, including how they are designed and the way in which they intake air and exhaust gases, is the same, but the real differences come into play when it comes to such things as the combustion temperature and pressure, operating RPM, and driving cycle. It’s in those areas that diesel and petrol engines are pretty far apart, so let’s take a look at the two and see how those factors come into play when a turbocharger is added to the mix.
Everything is a little larger and stronger in a diesel engine so that they can handle the high combustion pressures caused by the diesel fuel being atomized when it enters the combustion chamber. All that extra size makes it harder for diesel engines to operate well at higher RPM’s, which in turn means that they can’t draw in a lot of air to generate that extra power. This is where a turbocharger can really help a diesel engine, although it has to be properly sized to make sure that it delivers on the extra power promise.
Diesel fuel burns at very low temperatures which mean that it creates a lot more exhaust gases than a petrol engine. What that means is that you have to fit your engine with a turbo that has a large turbine section to accommodate the entire exhaust gas and air intake. Given that diesel engines already run high compression, the power boost that you can expect to get from the addition of a turbocharger is typically quite low (5-8 psi). It’s also worth noting that the spool up time for a turbo engine is relatively slow which is actually fine since most diesel turbo engines are installed for dependability and reliable engine running rather than for pure power.
If diesel fuel is the slow burn member of the family, petrol is the hot-tempered volatile brother that burns at an intense heat. Unlike the low RPM’s of diesel, petrol engines need to operate at a wide variety of RPM’s, to make them drivable. The addition of a turbocharger in a petrol engine is all about power, which in this case means that the turbo has to spin at an incredibly high rate of speed in order to achieve that. Given that driving a petrol vehicle means moving up and down the gears, a quicker spool up time is required in order to make that happen. It’s also important that the back pressure is paid attention to in a petrol engine so that there is no damage, and let’s not forget about the heat generated in such an engine. A cooler of some kind has to be attached to the turbo so that the engine won’t run hot. As you can see, diesel and petrol turbochargers achieve the same thing, but with a few subtle differences along the way.