The air pressure used when starting a diesel engine with injected compressed air is approximately:

• A. 520 kPa
• B. 255 kPa
• C. 2100 kPa
• D. 3030 kPa

Answer: (C)

Starting a diesel with injected compressed air needs enough pressure to push the piston and overcome the cylinder’s compression and friction as the engine is cranked. That requires a fairly high pressure, typically around 20 bar (about 2100 kPa). This level provides sufficient force to fill and drive the cylinders until the fuel can ignite and the engine begins to run on its own.

Lower pressures like 5 bar or a few bar simply won’t provide enough torque to turn the engine over and compress the air inside the cylinders. A pressure around 30 bar is higher than usually necessary for many engines and can exceed what the starting system is designed to safely handle. So the standard starting-air pressure you’ll see cited for injected compressed-air starts is about 2100 kPa (approximately 21 bar).