MNC Engineering LLC

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THE OLD WAY                                                                                                                                         

In a standard four stroke, spark ignited engine a series of pistons, in a line, move up and down in cylinder sleeves capped by a header. The pistons connect to a crankshaft, via connecting rods, which controls the piston’s motion (stroke). 

In the first stroke (intake), the crankshaft pulls the piston down, from top dead center (TDC) crank angle zero (0 degrees CA) which enlarges the combustion chamber at the top of the piston (crown), and with the intake valve open sucks in the air/fuel mixture (AFM). At bottom dead center (BDC) the combustion chamber is at its largest volume (displacement) and the intake valve closes trapping in the ingested AFM. So far the crankshaft has rotated ½ of a revolution (180 degrees CA) and the second stroke (compression) begins pushing the piston up the sleeve, back to TDC (360 degrees CA). This action compresses and heats the AFM according to the physical parameters of the engine and is called its compression ratio (CR). Since the pistons’ up and down strokes are controlled by the crankshaft, they are all exactly the same length, and the compression ratio is set as a fixed value during the engine’s design phase. Secondly, compressing the AFM takes a surprising amount of energy which is referred to as pumping losses. But it gets worse. To get the maximum amount of energy out of this engine the AFM burn must be completed at about 20 degrees after TDC (380 degrees CA) so it starts at about 20 degrees before TDC (340 degrees CA) since it takes that long for the burn to complete (milliseconds or about 40 degrees CA). Thus the AFM is burning before TDC and it’s getting hotter and its pressure is rising over and above that caused by the compression process itself which increases your pumping losses. This is what’s referred to as negative work.

By now the piston has reached TDC (360 degrees CA), and the third stroke (expansion) has officially begun. Its volume is about 1/11th of its previous size (CR 11:1), its pressure is around 700 psi and its temperature is about 2200 degrees F. Diesels operate at CR’s of about 18:1 which makes them more fuel efficient, but gas engines are limited in this area because of a phenomenon called knock. This occurs because a hot spot in the combustion chamber can prematurely ignite the AFM or the compression ratio is so high that the ignition temperature of the AFM is reached through the normal combustion process itself. These are untimed events that don’t do the engine much good. This is the only time that the engine actually produces any work, all the other strokes are strictly maintenance to get to this point, and they all cost energy. We are now at the fourth and final stroke (exhaust). Here the exhaust valve opens and the still hot and under pressure exhaust gases exit the engine. The upward movement of the piston back to TDC, another ½ turn of the crankshaft, (720 degrees CA) facilitates this process, and the four stroke cycle is ready to begin again. But did you notice that the exhaust gases were still under pressure when the exhaust valve opened, and that gas pressure was just completely wasted? And that during combustion the temperatures are so high that we need cooling to keep surfaces within reasonably temperatures?

In principle, that’s it. There are literally hundreds of details that have been skipped over which can have a significant impact on performance, but for those, you’ll have to buy a book on engine thermodynamics. So what have we learned?

  • There is only so much energy in a gallon of gas and we’re wasting most of it.
  • Even an ideal engine will only recover another third of it, and no more.
  • It takes 2 revolutions of the crankshaft to produce ½ revolution of power.
  • The piston strokes (all 4) are of equal and fixed length.
  • A water cooling system with a radiator is required to cool the engine down.
  • While timing is important, compression ratios are basically everything; the higher, the more efficient the engine, and a few terms used in the trade.