**2.2 Compression stroke**

This is the period when the piston moves up from BDC to TDC, and compresses the air inside the cylinder to a small fraction of the total cylinder volume. During this time, the intake and exhaust valves are closed; and as the piston approaches the TDC, diesel fuel is injected into the hot compressed air at a temperature sufficiently high to ignite the fuel, which burns and releases high thermal energy.

### **2.3 Power or expansion stroke**

The expansion of high temperature and pressure burning gases pushes the piston down from the TDC to BDC giving the useful power stroke. This is also called the work stroke because the high pressure gases act on top of the piston surface. Its impact on the connecting rod is transmitted to the crankshaft causing it to rotate and produce useful work on the shaft. During this process, both valves are theoretically closed between the centers but practically, the exhaust valve is timed to open some degrees before BDC to initiate pressure drop inside the cylinder by allowing burnt gases to escape.
