Diesel Engines

Modern road diesels use compression ratios of about 15:1 to 18:1 (older indirect-injection engines ran higher). Near top dead centre the trapped air reaches several hundred degrees Celsius, above the auto-ignition temperature of diesel fuel. Fuel injected at that moment evaporates and ignites after a short ignition delay; fuel with a higher cetane number ignites more readily. Fuel that accumulates during the delay burns suddenly, which causes the characteristic diesel knock, so common-rail systems fire one or more small pilot injections first to start combustion gently before the main injection.

Load is controlled by fuel quantity rather than air quantity ('quality control'), so the engine runs overall lean — λ is well above 1 at part load — and avoids throttling losses. High compression gives a high expansion ratio, extracting more work from each charge. Peak efficiency of a modern passenger-car diesel is typically around 40–45%. Nearly all are turbocharged because a diesel can accept boost without knock; the limit is smoke, peak cylinder pressure and exhaust temperature.

A common-rail system stores fuel at roughly 1,600–2,500 bar in a shared rail; solenoid or piezo injectors open several times per cycle under ECU control. Because combustion happens in locally rich and locally hot zones, the engine produces particulates and NOx. After-treatment typically includes a diesel oxidation catalyst (CO, HC), a diesel particulate filter that traps soot and periodically burns it off (regeneration), exhaust gas recirculation to lower combustion temperatures, and selective catalytic reduction where injected urea forms ammonia that converts NOx to nitrogen and water.