Four-Stroke vs Two-Stroke IC Engines: Working and Power Calculation
An internal combustion (IC) engine burns fuel inside the cylinder and uses the expanding gas to push a piston. The same basic parts, piston, cylinder, connecting rod and crankshaft, appear in a motorcycle, a car, a tractor and a generator set. The difference between a four-stroke and a two-stroke engine lies in how many piston strokes it takes to complete one cycle of intake, compression, power and exhaust. This article explains both, compares them, and then calculates the key performance figures for a small single-cylinder engine.
How a Four-Stroke Engine Works
A four-stroke engine completes one cycle in four strokes of the piston, which is two revolutions of the crankshaft. Poppet valves operated by a camshaft running at half crank speed control the flow of gases.
- Suction (intake): piston moves from top dead centre (TDC) to bottom dead centre (BDC), the inlet valve is open and fresh charge enters.
- Compression: both valves are closed and the piston moves up, compressing the charge.
- Power (expansion): near TDC the spark plug fires (petrol) or fuel is injected into hot air (diesel); the burning gas forces the piston down.
- Exhaust: the exhaust valve opens and the rising piston pushes burnt gas out.
In real engines the valves do not open and close exactly at TDC and BDC. They open early and close late to use the momentum of the gas, and for a short period around TDC both valves are open together. This is called valve overlap and is shown on a valve timing diagram.
How a Two-Stroke Engine Works
A two-stroke engine completes the cycle in one crankshaft revolution. In the common crankcase-scavenged petrol design there are no valves; ports in the cylinder wall are uncovered by the piston itself.
- Upward stroke: the piston compresses the charge above it while creating suction in the sealed crankcase below, drawing in fresh mixture through the inlet port or reed valve.
- Downward stroke: after ignition the piston is pushed down, first uncovering the exhaust port so burnt gas escapes, then the transfer port so the pre-compressed charge in the crankcase flows into the cylinder and pushes out the remaining exhaust (scavenging).
Because the exhaust and transfer ports are open together, some fresh charge escapes with the exhaust. This short-circuiting is the main reason small two-stroke petrol engines have higher fuel consumption and hydrocarbon emissions, and why they have largely disappeared from Indian two-wheelers. Large marine diesel engines, however, still use the two-stroke cycle with separate scavenge air blowers and exhaust valves, and are among the most efficient engines built.
Comparison
| Feature | Four-stroke | Two-stroke (small petrol) |
|---|---|---|
| Power strokes | One every 2 revolutions | One every revolution |
| Valves | Poppet valves with camshaft | Ports in cylinder wall |
| Power for same size | Lower | Higher in theory, up to about twice |
| Fuel efficiency | Better | Poorer due to charge loss |
| Lubrication | Separate oil sump | Oil mixed with fuel or injected |
| Emissions | Lower | Higher hydrocarbons and smoke |
| Construction | More parts | Simple and light |
| Typical use | Cars, bikes, generators | Chainsaws, small outboards, older scooters |
Performance Formulas
Swept volume Vs = (pi/4) * D^2 * L
Compression ratio r = (Vs + Vc) / Vc
Indicated power IP = pm * L * A * n * k / 60 (W, pm in Pa)
n = N/2 for four-stroke, n = N for two-stroke
k = number of cylinders
Mechanical efficiency eta_m = BP / IP
Brake thermal eff. eta_bt = BP / (m_fuel * CV)
Mean piston speed Vp = 2 * L * N / 60Worked Example: Small Single-Cylinder Engine
A single-cylinder four-stroke petrol engine has a bore of 50 mm, stroke of 49.5 mm and clearance volume of 10.8 cc. At 6000 rpm the indicated mean effective pressure is 8 bar and mechanical efficiency is 80 percent. Fuel consumption is 1.2 kg/h with calorific value 44,000 kJ/kg.
Vs = pi/4 * 5.0^2 * 4.95 = 97.2 cc r = (97.2 + 10.8) / 10.8 = 10.0 A = pi/4 * 0.05^2 = 1.9635e-3 m^2 n = 6000 / 2 = 3000 power strokes per minute IP = 8e5 * 0.0495 * 1.9635e-3 * 3000 / 60 = 3888 W = 3.89 kW BP = 0.80 * 3.89 = 3.11 kW Heat supplied = 1.2 * 44,000 / 3600 = 14.67 kW eta_bt = 3.11 / 14.67 = 21.2 % Mean piston speed = 2 * 0.0495 * 6000 / 60 = 9.9 m/s
If the same cylinder ran on a two-stroke cycle with the same mean effective pressure, n would equal N and indicated power would double to about 7.8 kW. In practice it is lower because part of the stroke is lost to port timing and the trapped charge is smaller, but it shows why two-strokes have a high power-to-weight ratio.
Common Mistakes
- Using N instead of N/2 for a four-stroke engine, which doubles the calculated power.
- Entering mean effective pressure in bar instead of pascal (1 bar = 100,000 Pa).
- Calculating compression ratio as Vs/Vc instead of (Vs + Vc)/Vc.
- Confusing indicated power (inside the cylinder) with brake power (at the crankshaft).
Summary
Four-stroke engines spend two revolutions per cycle and use valves; two-stroke engines complete the cycle in one revolution using ports. With bore, stroke, speed and mean effective pressure you can calculate swept volume, compression ratio, indicated and brake power and thermal efficiency. The ideal cycles behind these engines, Otto and Diesel, are analysed next.