Engine construction and operation

A four-stroke SI engine completes one power cycle in four piston strokes, i.e. two crankshaft revolutions (720 degrees).

 1 SUCTION    2 COMPRESSION  3 POWER      4 EXHAUST
 IV open      both closed    both closed  EV open
 |  |v| |     |  |  |  |     |  |* |  |   |  |  |^|
 |_ | |_|     |_ |  |_ |     |_ |  |_ |   |_ |  | |
  | |            |||           |||           | |
  piston down    piston up     piston down   piston up
  mixture in     mixture       spark,        burnt gas
                 compressed    expansion     out

1. Suction: the piston moves from TDC to BDC, the inlet valve is open and a petrol-air mixture from the carburettor or injector enters.
2. Compression: both valves are closed, the piston rises and compresses the mixture (r = 8-10). Near the end the spark plug ignites it.
3. Power (expansion): the burning gas pushes the piston down. This is the only stroke giving work.
4. Exhaust: the exhaust valve opens and the rising piston pushes out the burnt gases.

Major components and functions

Materials: cast iron or aluminium alloy for block and piston, forged steel for the crankshaft and connecting rod.

Both engines are reciprocating internal combustion engines. An SI engine ignites a premixed fuel-air charge with a spark. A CI engine compresses air only and injects fuel into it near the end of compression, when it self-ignites.

Use: SI engines in cars and motorcycles; CI engines in trucks, buses, tractors, generators and ships.

Two-stroke engine working

A two-stroke engine completes the cycle in two piston strokes (one revolution). It has ports in the cylinder wall instead of valves: the inlet port, the transfer port and the exhaust port. The crankcase acts as a pump.

   spark plug
      |
  +---+---+
  |       |   exhaust port --> E
  |  [P]  |
  |       |   transfer port <-- T
  |_______|
  | crank |<-- inlet port (reed valve) <-- mixture
  +-------+

1. Upward stroke: the piston covers the transfer and exhaust ports and compresses the charge above it. At the same time the rising piston lowers the crankcase pressure and fresh mixture is drawn in through the inlet port. Near TDC the spark ignites the charge.
2. Downward stroke: the burning gas expands and pushes the piston down (power). The piston first uncovers the exhaust port, so burnt gas leaves. As it moves further it compresses the mixture in the crankcase and uncovers the transfer port, so the fresh charge flows into the cylinder and pushes out the remaining exhaust gas (scavenging). A deflector on the piston directs the flow upward.

Comparison

Given: $D = 0.1$ m, $L = 0.12$ m, $z = 4$, $r = 9$, $N = 2400$ rpm.

(a) Swept volume and displacement

$$\begin{aligned} V_s &= \frac{\pi}{4} D^2 L = \frac{\pi}{4}(0.1)^2(0.12) = 9.425 \times 10^{-4}\ \text{m}^3 = 942.5\ \text{cm}^3 \\ V_{engine} &= z V_s = 4 \times 942.5 = 3770\ \text{cm}^3 = 3.77\ \text{L} \end{aligned}$$

(b) Clearance volume

$$r = \frac{V_c + V_s}{V_c} \Rightarrow V_c = \frac{V_s}{r - 1} = \frac{942.5}{8} = 117.8\ \text{cm}^3$$

(c) Mean piston speed

The piston travels $2L$ in each revolution:

$$\bar{S}_p = 2LN = 2 \times 0.12 \times \frac{2400}{60} = 9.6\ \text{m/s}$$

(d) Power strokes per second

In a four-stroke engine each cylinder fires once in two revolutions:

$$\text{Firing rate} = z \times \frac{N}{2 \times 60} = 4 \times \frac{2400}{120} = 80\ \text{per second}$$

Answer: (a) $V_s = 942.5$ cm$^3$, displacement 3.77 L; (b) $V_c = 117.8$ cm$^3$; (c) 9.6 m/s; (d) 80 power strokes per second for the engine (20 per cylinder).