Unlike Petroleum ether and petrol engines, diesel engines do not use high
voltage spark ignition (spark plugs). An engine running on diesel compresses
the air inside the cylinder to high pressures and temperatures (compression
ratios from 15:1 to 21:1 are very common); the diesel is generally injected
directly into the cylinder near the end of the compression stroke. (Hence
TDI – Turbo Direct Injection). The high temperatures inside the cylinder
causes the diesel fuel to react with the oxygen in the mix (burn or
oxidize), heating and expanding the burning mixture in order to convert the
thermal/pressure difference into mechanical work; i.e., to move the piston.
(Glow plugs are used to assist starting the engine to preheat cylinders to
reach a minimum operating temperature.) High compression ratios and
throttle-less operation generally result in diesel engines being more
efficient than many spark-ignited engines.
This and being less flammable and explosive than gasoline are the main
reasons for military use of diesel in armoured fighting vehicles like tanks
and trucks. Engines running on diesel also provide more torque and are less
likely to stall as they are controlled by a mechanical or electronic
governor. A disadvantage of diesel as a vehicle fuel in some climates,
compared to gasoline or other petroleum derived fuels, is that its viscosity
increases quickly as the fuel's temperature decreases, turning into a
non-flowing gel at temperatures as low as -19 °C (-2.2 °F) or -15 °C (+5
°F), which can't be pumped by regular fuel pumps. Special low temperature
diesel contains additives that keep it in a more liquid state at lower
temperatures, yet starting a diesel engine in very cold weather still poses
considerable difficulties.
Diesel powered cars generally have a better fuel economy than equivalent
petrol engines and produce less greenhouse gas emission.
The greater economy of the Diesel engines is due to the higher energy
per-litre content of diesel fuel and the intrinsic efficiency of the diesel
engine.
While petrodiesel's higher density results in higher greenhouse gas
emissions per litre compared to petrol, the 20–40% better fuel economy
achieved by modern diesel-engined vehicles offsets the higher-per-litre
emissions of greenhouse gases, and produces 10-20 percent less greenhouse
gas emissions than comparable petrol engined vehicles.
Biodiesel (Bio Diesel) diesel engines offer substantially improved emission
reductions compared to petro-diesel or petrol powered engined cars, while
retaining most of the fuel economy advantages over conventional
gasoline-powered automobiles.
However, the increased compression ratios mean that there are increased NOx
emissions from diesel engines. This is compounded by biological nitrogen in
biodiesel to make NOx emissions the main drawback of diesel versus petrol
engines.