AJP engine - Is this true?
Discussion
I was told by a friend that the AJP engine runs as a normal V8 at idle, blatting each piston in turn, and that is why it sounds like an old school V8 at idle. Apparently though, when you increase the revs, the engine's ECU tells each set of pistons to fire two at a time, so you effectively get a really big straight 4. is this true?
FUBAR said:
I like this qoute "The sound of flat-plane V8 is usually somewhat like a pair of four-pot engines screaming simultaneously, unlike the rumble of cross-plane V8s. There have been comments like "It doesn't sound like a V8!!"....... but this should really be "It sounds like a Racing V8!"
longbow said:
No - the pistons move together in pairs relative to the crankshaft, but they will not be at the same point in the combustion cycle i.e. one will be on compression while the other is exhausting - just like an inline 4. Therefore they don't fire together.
Thats what OP's friend was thinking too, but i've just thought how it works now
Edited by WorAl on Thursday 22 October 13:28
on one bank you have 2 up and 2 down on the other you have 2 nearly up (15 deg off) and 2 nearly down (15 deg off). The result is that no you don't get two firing at the same time but the period between firings is uneven to the tune of +15 deg then - 15deg between banks. This is due to the 75 degree V angle (90-75 = 15).
WorAl said:
longbow said:
No - the pistons move together in pairs relative to the crankshaft, but they will not be at the same point in the combustion cycle i.e. one will be on compression while the other is exhausting - just like an inline 4. Therefore they don't fire together.
In an in-line 4 you have 2 up 2 down (1 firing, 1 exhausting, 1 expanding, 1 intaking - relative) 180 degrees apart. in the cross plane crank you have 2 up, 2 down. (1 firing, 1 exhausting, 1 expanding, 1 intaking) then 4 at 90 degrees to the other 4 (i.e. 2 left, 2 right, in an imaginary kind of way), half way through each action, so to speak. how does the firing order hapen in a flat plane crank then? you cant have 4 half way through the cycle like cross plane cranks. Its got to be one from each bank firing symultaneously or 2 from same bank (i.e. 2 at a time) or an 8 stroke engine In an inline 4, each cylinder fires once every two turns. Two cylinders fire in each turn of the engine, normally at 180 degree intervals.
Now take the flat plane V8. The big ends of the pistons are still at 180 degree intervals, across the centre of the crank. If the engine was a straight eight, with two pistons on each big end bearing as shown in that diagram, then there would be two cylinders firing simultaneously - say one on the front and back, then two middle ones, then the other two front and back, then the other two middle ones.
If the V was 180 degrees (ie boxer-style) then the same would apply - the pistons would reach the top and bottom of their strokes simultaneously (although the firing order wouuld be different - you would always have an outer cylinder on one side firing simultaneously with an inner on the other side).
You could configure the pistons to fire together as you liked, depending on the cam profiles and order of the ignition leads, but there would always have to be 2 at the same time.
Now, in the AJP, the pistons aren't in the same plane as the crank - the V is an acute angle (can't remember exactly what the angle is but it doesn't matter). As a result, the pistons on one side of the engine are always going to be in a different "phase" from the other side, by the angle of the Vee. They will be in the same relationship to each other, but not to the pistons on the other side of the V.
That means that you can't get 2 pistons firing at the same time.
The difference in sound from an "ordinary" V8 is because the flat plane V8 fires left-right-left-right-left-right-left-right so each exhaust carries even "bubbles" of pressure, whereas a cross plane crank engine fires left-right-right-left-right-left-left-right - ie two cylinders on each exhaust fire one after the other - the second one gives that "burble" while the first one sounds like a straight 4 beside another straight 4.
ETA and Fatjon said it in far fewer words than me!
Edited by tvrgit on Thursday 22 October 13:53
tvrgit said:
WorAl said:
longbow said:
No - the pistons move together in pairs relative to the crankshaft, but they will not be at the same point in the combustion cycle i.e. one will be on compression while the other is exhausting - just like an inline 4. Therefore they don't fire together.
In an in-line 4 you have 2 up 2 down (1 firing, 1 exhausting, 1 expanding, 1 intaking - relative) 180 degrees apart. in the cross plane crank you have 2 up, 2 down. (1 firing, 1 exhausting, 1 expanding, 1 intaking) then 4 at 90 degrees to the other 4 (i.e. 2 left, 2 right, in an imaginary kind of way), half way through each action, so to speak. how does the firing order hapen in a flat plane crank then? you cant have 4 half way through the cycle like cross plane cranks. Its got to be one from each bank firing symultaneously or 2 from same bank (i.e. 2 at a time) or an 8 stroke engine In an inline 4, each cylinder fires once every two turns. Two cylinders fire in each turn of the engine, normally at 180 degree intervals.
Now take the flat plane V8. The big ends of the pistons are still at 180 degree intervals, across the centre of the crank. If the engine was a straight eight, with two pistons on each big end bearing as shown in that diagram, then there would be two cylinders firing simultaneously - say one on the front and back, then two middle ones, then the other two front and back, then the other two middle ones.
If the V was 180 degrees (ie boxer-style) then the same would apply - the pistons would reach the top and bottom of their strokes simultaneously (although the firing order wouuld be different - you would always have an outer cylinder on one side firing simultaneously with an inner on the other side).
You could configure the pistons to fire together as you liked, depending on the cam profiles and order of the ignition leads, but there would always have to be 2 at the same time.
Now, in the AJP, the pistons aren't in the same plane as the crank - the V is an acute angle (can't remember exactly what the angle is but it doesn't matter). As a result, the pistons on one side of the engine are always going to be in a different "phase" from the other side, by the angle of the Vee. They will be in the same relationship to each other, but not to the pistons on the other side of the V.
That means that you can't get 2 pistons firing at the same time.
The difference in sound from an "ordinary" V8 is because the flat plane V8 fires left-right-left-right-left-right-left-right so each exhaust carries even "bubbles" of pressure, whereas a cross plane crank engine fires left-right-right-left-right-left-left-right - ie two cylinders on each exhaust fire one after the other - the second one gives that "burble" while the first one sounds like a straight 4 beside another straight 4.
ETA and Fatjon said it in far fewer words than me!
(no matter how many times I read these Vizzard books, I still don't understand them - I think I need an engine in front of me)
tvrgit said:
WorAl said:
longbow said:
No - the pistons move together in pairs relative to the crankshaft, but they will not be at the same point in the combustion cycle i.e. one will be on compression while the other is exhausting - just like an inline 4. Therefore they don't fire together.
In an in-line 4 you have 2 up 2 down (1 firing, 1 exhausting, 1 expanding, 1 intaking - relative) 180 degrees apart. in the cross plane crank you have 2 up, 2 down. (1 firing, 1 exhausting, 1 expanding, 1 intaking) then 4 at 90 degrees to the other 4 (i.e. 2 left, 2 right, in an imaginary kind of way), half way through each action, so to speak. how does the firing order hapen in a flat plane crank then? you cant have 4 half way through the cycle like cross plane cranks. Its got to be one from each bank firing symultaneously or 2 from same bank (i.e. 2 at a time) or an 8 stroke engine In an inline 4, each cylinder fires once every two turns. Two cylinders fire in each turn of the engine, normally at 180 degree intervals.
Now take the flat plane V8. The big ends of the pistons are still at 180 degree intervals, across the centre of the crank. If the engine was a straight eight, with two pistons on each big end bearing as shown in that diagram, then there would be two cylinders firing simultaneously - say one on the front and back, then two middle ones, then the other two front and back, then the other two middle ones.
If the V was 180 degrees (ie boxer-style) then the same would apply - the pistons would reach the top and bottom of their strokes simultaneously (although the firing order wouuld be different - you would always have an outer cylinder on one side firing simultaneously with an inner on the other side).
You could configure the pistons to fire together as you liked, depending on the cam profiles and order of the ignition leads, but there would always have to be 2 at the same time.
Now, in the AJP, the pistons aren't in the same plane as the crank - the V is an acute angle (can't remember exactly what the angle is but it doesn't matter). As a result, the pistons on one side of the engine are always going to be in a different "phase" from the other side, by the angle of the Vee. They will be in the same relationship to each other, but not to the pistons on the other side of the V.
That means that you can't get 2 pistons firing at the same time.
The difference in sound from an "ordinary" V8 is because the flat plane V8 fires left-right-left-right-left-right-left-right so each exhaust carries even "bubbles" of pressure, whereas a cross plane crank engine fires left-right-right-left-right-left-left-right - ie two cylinders on each exhaust fire one after the other - the second one gives that "burble" while the first one sounds like a straight 4 beside another straight 4.
ETA and Fatjon said it in far fewer words than me!
Edited by tvrgit on Thursday 22 October 13:53
- Brain jumps out of head*
Erm right ok, I have SOME understanding of it, but without seeing it infront of me I'm having trouble imagining it. I do however now understand why my dad's 4.5 (and also to a lesser degree my V6) seem to have an irregular exhaust rhythmn, where one side will fire twice to the other side's once. This explains it.
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