Automotive Engines, Fuels, Emissions and Electrical Systems
Engine fundamentals
A four-stroke SI engine runs suction, compression, power and exhaust in two crank revolutions; the CI engine inducts air only and injects fuel near TDC. Compression ratio CR = (Vs + Vc)/Vc. Petrol quality is rated by octane number (iso-octane 100, n-heptane 0; research and motor methods averaged), diesel by cetane number (n-hexadecane 100). High octane resists SI knock; high cetane shortens ignition delay and reduces diesel knock.
Combustion stages: in SI engines - ignition lag, flame propagation, afterburning; in CI engines - ignition delay, premixed (uncontrolled) burning, mixing-controlled burning, late burning. Diesel knock is governed by ignition delay and the fuel's cetane rating.
Fuels, emissions, electronics
- Modern petrol systems: MPFI; modern diesel: common-rail direct injection with high rail pressure and pilot injection.
- Harmful emissions: CO, HC, NOx and particulates. A three-way catalytic converter needs stoichiometric operation (λ = 1) to oxidise CO/HC and reduce NOx together; it cannot do both on a lean diesel.
- Diesel after-treatment: DOC + DPF + SCR with AdBlue (urea) to cut NOx; EGR lowers combustion temperature and therefore NOx.
- Bharat Stage VI took effect on 1 April 2020, skipping BS-V, and caps sulphur at 10 ppm.
- Electrical: 12 V lead-acid battery of six cells at about 2.1 V each; alternator with built-in rectifier and regulator; starter motor; ECU inputs from MAP/MAF, lambda, crank and cam position, and throttle sensors over a CAN bus.
Worked example
Bore 80 mm, stroke 90 mm, clearance volume 50 cc. Vs = (π/4) × 8² × 9 = 452.4 cc. CR = (452.4 + 50) / 50 = 10.05 : 1.
Traps
- Treating octane as an energy measure; it is an anti-knock index only.
- Swapping cetane and octane scales.
- Assuming a three-way catalyst works on a lean-burn diesel.