Diesel engines have much higher compression, and that means they have to have much tighter tolerances everywhere; smoother cylinder bores, better valve seats, better valves. This used to make the engine much heavier, but doesn’t so much these days due to the prevalence of turbo petrol engines; the block for a turbo engine weighs about the same whether it is petrol or diesel, and in fact some manufacturers use the exact same casting.
Diesels also run much hotter, which means better materials and better oil.
They run much higher fuel pressures than most petrol engines due to the way their fuel injection works, which again means tighter tolerances and better materials.
To protect those materials and tolerances, they need much better fuel and air filters.
They’re usually turbocharged, and the turbos spend more of their time at high RPM because diesels mostly don’t have throttles, so they run at comparatively very high air mass flow all the time. That high air mass also makes the air filter’s job harder again. The high RPM makes the turbo bearings harder to lubricate. The turbo also has to deal with exhaust gas several hundred degrees hotter, and a lot more of it. Of course, that means the turbo works a lot better, which is why it’s always there on a modern engine… turbos are cheaper than bigger engines, and make the engine more efficient. A lot more efficient than it is possible for a petrol engine to be, actually, thermodynamics guarantees this; turbodiesels can have an efficiency up around 50%, no petrol engine can ever exceed something like 36%.
The high air flow means that the intercooler needs to be big, which means finding somewhere to put it and the huge hoses that connect it to the engine.
Diesels produce their power and torque at lower RPM, and have MUCH more torque anyway, which means bigger, heavier and more expensive transmission components.
Diesels make more noise and vibrate more, which means more sound damping material and bigger and better engine mounts. Fortunately, they make a lot less exhaust noise, so the mufflers and exhaust system can be smaller and lighter.
So, why would you buy one?
Simple: they’re really quite awesome to drive due to the massive torque at low RPM, and use less fuel. That doesn’t make them much cheaper to run due to the higher service costs caused by those filters and the fairly expensive oil that can stand the high temperatures in the turbo and heads, but it does mean they go further on a tank. A lot further. Yes, the higher vibration level is a bit annoying in heavy traffic, but cruising you don’t notice.
Contributed by Pro PavanRaj Urs, Bangalore, ARPRO 3