We have discussed the Air-standard cycles and the actual engines such as the spark-ignition and compression ignition engines. But the efficiency of these actual cycles is always less than the air-standard cycles. We have discussed the comparison between the Air-standard cycles and the Actual cycles and listed the most influential factors such as the Time Loss Factor, Heat Loss Factor, Exhaust Blowdown. In this article, we are going to discuss the Exhaust Blowdown.
Blowdown is nothing but the escape of the combustion particles from the cylinder when the exhaust valve opens. The timing of the opening of the exhaust valve will also have an influence on the efficiency of the engine since there is this Exhaust blown loss during the cycle. This loss depends on the timing of the
As we can see the following P-V diagram of a petrol engine with all three losses (Time Loss, Heat Loss and Exhaust Loss) with the occupied percentages of loss during the cycle. It is noted that the loss of this exhaust blown is 2%. Which is comparatively less than the other two losses (Time loss-6% and Heat Loss-12%) which means it will have less effect on the efficiency of the engine.
But This Exhaust Loss also has to be considered to improve the overall efficiency of the engine.
- Generally, the cylinder pressure at the end of the expansion stroke is about 7 bar depending on the compression ratio employed.
- If the exhaust valve is opened at the Bottom dead centre then the piston has to do the work against the cylinder pressure due to the early part of the exhaust stroke. See the dashed line representation in the above P-V diagram.
- If the Exhaust valve is opened too early, then some portion of the expansion stroke is lost.
- The optimum duration to open the exhaust valve is 40° to 70° before BDC thereby reducing the cylinder pressure to halfway (let’s say 3.5 bar) before the exhaust stroke begins.
This is how an exhaust blowdown occurs. The optimum timing of valve opening will improve the efficiency of the engine that has been lost by the exhaust blow down.
We have discussed the Exhaust Blowdown in the actual engines along with the optimum position to open the exhaust valve. If you have any further thoughts about this topic, let us know in the comment section below.