KevinK2
Mechanical
- Feb 6, 2012
- 119
I searched and could not find an ASTM, ASME, SAE, or DIN standard defining displacement.
My research lead me back to early "piston pumps" (predating steam engines). There displacement was defined as # of pistons x piston displacement, as these pumps worked on a 1 rev cycle.
I think this method was copied by the otto cycle engines, even though they needed 2 revs to complete a cycle.
When 2 cycle engines appeared, the same "piston pump" approach to displacement was used, but these engines need just one rev to complete all cycles. This advantage was handled in motorcycle racing by allowing about 50-70% more displacement for the 4 stroke bikes, that competed against the 2 stroke bikes.
With the 2 and 4 stroke engines, the "piston pump" rule could always be substantiated by considering each engine as a 1 rev pump to verify displacement.
Enter the Wankel.
The "1.3L" engine found today in Mazdas was very different, with dynamic combustion chambers, and 3 revs to complete a single chambers's 4 strokes. The developers apparently used a different rule to get displacement. They said you have 2 rotors, and each rotor fires once PER REV, a fired chamber is .64L, so displacement is 2 rotors x .64L per rotor = 1.3L displacement ... PER REV.
With this logic, and rotor = piston, a 2 stroke would have the correct displacement (per rev) for 100% efficency. The 4 stroke 5.0L engine would be 2.5L, based on Wankel's "1 rev" method. This is actually a good way to determine displacement, as it is consistent for all engines; 100% VE ingestion per rev.
I don't think anyone is prepared to call their 5.0 V8 a 2.50. So, if one were to apply the "piston pump" rule to the Wankel:
Each rotor has 3 distinct combustion chambers per rotor, that take 3 output shaft revs to fire. With 2 rotors, that's 3.8L fired in 3 revs.
As a wankel pump, each of the sets of 4-otto-cycles of a combustion chamber cycle, provide 2 pumping cycles for a pump (note that the valving and porting are ignored in the basic "piston pump" displacemnt calc). So you take 2 x the fired chambers (pumping strokes) and divide by 3 to get one rev, and you have 2.6L displacement rating, per the "piston pump" method. This is actually the number of chambers that fire in 2 revs, and is an equal basis for rating the extremely common 4 stroke piston engine.
Other than the "piston pump" method, is there any official standard for displacement?
Kevin
My research lead me back to early "piston pumps" (predating steam engines). There displacement was defined as # of pistons x piston displacement, as these pumps worked on a 1 rev cycle.
I think this method was copied by the otto cycle engines, even though they needed 2 revs to complete a cycle.
When 2 cycle engines appeared, the same "piston pump" approach to displacement was used, but these engines need just one rev to complete all cycles. This advantage was handled in motorcycle racing by allowing about 50-70% more displacement for the 4 stroke bikes, that competed against the 2 stroke bikes.
With the 2 and 4 stroke engines, the "piston pump" rule could always be substantiated by considering each engine as a 1 rev pump to verify displacement.
Enter the Wankel.
The "1.3L" engine found today in Mazdas was very different, with dynamic combustion chambers, and 3 revs to complete a single chambers's 4 strokes. The developers apparently used a different rule to get displacement. They said you have 2 rotors, and each rotor fires once PER REV, a fired chamber is .64L, so displacement is 2 rotors x .64L per rotor = 1.3L displacement ... PER REV.
With this logic, and rotor = piston, a 2 stroke would have the correct displacement (per rev) for 100% efficency. The 4 stroke 5.0L engine would be 2.5L, based on Wankel's "1 rev" method. This is actually a good way to determine displacement, as it is consistent for all engines; 100% VE ingestion per rev.
I don't think anyone is prepared to call their 5.0 V8 a 2.50. So, if one were to apply the "piston pump" rule to the Wankel:
Each rotor has 3 distinct combustion chambers per rotor, that take 3 output shaft revs to fire. With 2 rotors, that's 3.8L fired in 3 revs.
As a wankel pump, each of the sets of 4-otto-cycles of a combustion chamber cycle, provide 2 pumping cycles for a pump (note that the valving and porting are ignored in the basic "piston pump" displacemnt calc). So you take 2 x the fired chambers (pumping strokes) and divide by 3 to get one rev, and you have 2.6L displacement rating, per the "piston pump" method. This is actually the number of chambers that fire in 2 revs, and is an equal basis for rating the extremely common 4 stroke piston engine.
Other than the "piston pump" method, is there any official standard for displacement?
Kevin
