In terms of specs and raw data, petrol powered engines have their diesel counterparts whopped hard on pretty much everything. However, it is a well-known fact that diesel guzzlers unswervingly yield way more torque than petrol engines. The gap between BHP and torque, measured in lb ft, is a lot slimmer in diesel engines as well. What’s the secret to all that additional grunt? How does an underdog produce so much torque that it can change the earth’s rotation (metaphorically, of course)? Let’s dig in and find out.
Greater Compression Ratio The primary reason for all that extra kick is the high compression ratio. Simply put, diesel engines possess pistons that travel a greater distance inside the cylinder for each stroke. The volume of the internal space of the cylinder changes to a greater extent, resulting in a greater ratio of compression. This is also why diesel engines always rev lower than petrol ones. Petrol engines use shorter strokes to attain a high rev count and chunk out short bursts of power resulting in higher speeds. Diesel engines on the other hand expand that combusted power in heavier doses ensuing a more powerful rotation of the wheels. A perfect analogy to explain this would be carrying constructions material, say bricks, to a site. Petrol powered engines focuses on carrying each brick as fast as possible to the site while the diesel powered one carries four brick at a time but at a slower rate. The diesel engine is doing more work per engine rotation.
Higher Specific Energy Another factor that affects the torque output of an engine is the amount of force that is exerted on the piston from burning all that fuel. Diesel has a lower energy to mass ratio (45500kJ/kg) than petrol (45800kJ/kg), which shows that more heat energy is contained in petrol than diesel for a given volume. However, diesel happens to be much denser than petrol, so much so that, for a given volume, diesel can store up to 15% more heat energy. These numbers may seem gibberish but they play an important part in producing all that extra torque. Each time a fixed volume of diesel and petrol is burned, the diesel gives out more energy, which in turn is transferred to the pistons resulting in increased torque rushing through the crankshaft.
Higher boost In order to compensate for the low horsepower and speed, most vehicles possessing diesel engines comes turbo charged. This supplies air to the engine at a greater pressure than normal and gives a surge to the engine's power resulting in higher pressure within the cylinder. This simple redirection of air has substantial influence on increasing torque. In contrast to turbochargers in petrol engines, diesel engine turbos are commonly set to provide a much higher-than-stock boost pressure, allowing the engine to become more efficient in transferring the energy through to the rotating shaft. What all these jargons suggest is that horsepower isn’t everything, but it certainly has its uses though. A racing petrol-powered engine can produce very little torque but the utterly high rev count can give it a high horsepower. On the flip side, a diesel engine with low rev count can exploit on the torque gains but will be hard pressed to attain the same value in horsepower. This is primarily why we don’t see diesel engines in high-end and fancy hyper cars and their kin. It is worth mentioning that diesel engines were created to substitute steam engines; to rival engines designed to move heavy loads. To that end, high speed wasn’t a priority; a moderate speed with a huge torque was all that was expected off of these contraptions.High end functions were left to the gasoline powered engines, to provide high power to weight ratio and essentially producing that “quick throttle” characteristic.
The Alpina D3 Bi-Turbo, a relatively unknown vehicle boasts a BHP of 345 and 516 lb ft of torque; 0-60 mph in 4.6 seconds! That’s BMW M2 territory and the Alpina rose to the challenge, in spite of being a massive real estate of a car. What’s most astonishing about it is its engine makeup, it’s diesel-powered. Let’s not forget the Audi R8 V12 TDI, the 2008 production vehicle that could do a top speed of 202 mph, difficult to match even by today’s supercar standards. So, it seems diesel power can be utilised more efficiently and effectively to produce remarkable machineries, thanks in large part to the recent increase in demand for hybrids. However, it’d be daft to expect Ferrari, Lamborghini and other such high-end car manufacturers to flock to the diesel wagon. There is still some hope since in the end, power is power and “no” means too redundant, even if it means switching to a “dirty” diesel.