Engine and Powertrain Systems
Engine and Powertrain Systems
This domain tests four-stroke operation, engine displacement and compression, and how torque flows through the clutch, gearbox, final drive, and differential. Expect calculation and identification questions.
Core rules
- A four-stroke cycle needs 720 degrees of crankshaft rotation: intake, compression, power, exhaust. Valve overlap and variable valve timing change breathing.
- Spark-ignition petrol engines meter air and fuel near stoichiometric 14.7:1. Compression-ignition diesels run lean and control power mainly by injected fuel mass.
- Displacement per cylinder = pi/4 x bore squared x stroke. Use bore, not radius, then multiply by cylinder count.
- Compression ratio = (swept volume + clearance volume) / clearance volume.
- Wheel torque = engine torque x gear ratio x final-drive ratio x efficiency, roughly 0.85 to 0.95.
- Clutch, torque converter, CVT, and dual-clutch systems interrupt or multiply torque differently; use only the specified fluid.
Worked example
A four-cylinder engine has an 80 mm bore and 90 mm stroke. Convert to 8.0 cm and 9.0 cm. Piston area = pi/4 x 8.0 squared = 50.27 cm squared. One cylinder = 50.27 x 9.0 = 452.4 cm cubed. Four cylinders = 1,809.6 cm cubed, about 1.81 L. Reasoning: 80 mm is diameter; squaring the radius would understate displacement by four times.
Traps
- Using bore radius instead of bore diameter.
- Mixing cubic centimetres and litres in the final answer.
- Assuming peak power and peak torque occur at the same rpm.
- Ignoring driveline losses when calculating wheel torque.
- Using the wrong fluid in a CVT or dual-clutch gearbox.