How does a 4 stroke V8 work
Discussion
Ime timing a 4 cam V8,it has a crank locator and all 4 cams(2inlet and 2 exhaust) have to match with their locator holes,which i have done.
However the build video that i am watching sez you can be 360 degrees out,personaly i can not see this as if all pins line up then there's not any room for error.
If it helps the crank turns 2 revs to the cams 1 rev and on a downward stroke the exhaust opens ,on an upward stroke the intake opens.
Does this mean the spark only fires ever other crank revolution?
However the build video that i am watching sez you can be 360 degrees out,personaly i can not see this as if all pins line up then there's not any room for error.
If it helps the crank turns 2 revs to the cams 1 rev and on a downward stroke the exhaust opens ,on an upward stroke the intake opens.
Does this mean the spark only fires ever other crank revolution?
ZR1cliff said:
on a downward stroke the exhaust opens ,on an upward stroke the intake opens.
Does this mean the spark only fires ever other crank revolution?
Well you only need a spark every other revolution. The piston reaches TDC twice in the series of 4 strokes. One of these is the end of the compression stroke (when you want a spark), and the other is the end of the exhaust stroke (When you can have a spark...but you dont need it).
I cant think of a reason why you would open the exhaust valves on a downwards stroke. That would suck exhaust gasses into the cylinder. Exhaust valves open on the upwards stroke would allow the piston to push the exhaust gasses out.
www.keveney.com/otto.html
>> Edited by Munter on Tuesday 30th August 15:49
Munter said:
on a downward stroke the exhaust opens ,on an upward stroke the intake opens.
Does this mean the spark only fires ever other crank revolution?
Well you only need a spark every other revolution. The piston reaches TDC twice in the series of 4 strokes. One of these is the end of the compression stroke (when you want a spark), and the other is the end of the exhaust stroke (When you can have a spark...but you dont need it).
I cant think of a reason why you would open the exhaust valves on a downwards stroke. That would suck exhaust gasses into the cylinder. Exhaust valves open on the upwards stroke would allow the piston to push the exhaust gasses out.
www.keveney.com/otto.html
>> Edited by Munter on Tuesday 30th August 15:49
Thanks Munter,thats helped explain it to me,i got it wrong exhaust was upward stroke
Munter said:
I cant think of a reason why you would open the exhaust valves on a downwards stroke. That would suck exhaust gasses into the cylinder. Exhaust valves open on the upwards stroke would allow the piston to push the exhaust gasses out.
>> Edited by Munter on Tuesday 30th August 15:49
The exhaust valve is kept open during the downward stroke for better high rpm breathing:
As engine speeds increase there is the same amount of time around in terms of CRANK ANGLE but much much less in terms of real time in say, mili seconds.
It's the real time quantity that matters. The inertia of the compressible gas and tuning effects need REAL time hence the valve is kept open longer.
This is why real cams have valve OVERLAP with both inlet and exhaust open simultaneously for short periods- so that scavenging and exahust tuning can be facilitated.
Unfortunately at lower speeds when there IS alot of real time around these valve events will cause problems...... (potentially MORE on a cruciform crank V8!)
>> Edited by Marquis_Rex on Wednesday 31st August 13:15
During which "phase" does the overlap occur? Surly the valves are closed during the compression and explosion? At the end of the power stroke I can accept that the exhaust valves are open while the piston is still headed downwards. Therefore the overlap occures at/after the end of the exhaust stroke and into the intake stroke?
Munter said:
on a downward stroke the exhaust opens ,on an upward stroke the intake opens.
Does this mean the spark only fires ever other crank revolution?
Well you only need a spark every other revolution. The piston reaches TDC twice in the series of 4 strokes. One of these is the end of the compression stroke (when you want a spark), and the other is the end of the exhaust stroke (When you can have a spark...but you dont need it).
I cant think of a reason why you would open the exhaust valves on a downwards stroke. That would suck exhaust gasses into the cylinder. Exhaust valves open on the upwards stroke would allow the piston to push the exhaust gasses out.
www.keveney.com/otto.html
>> Edited by Munter on Tuesday 30th August
15:49
The exhaust valve must open bbdc because the gases are still expanding,if not you will lose power,the piston dont push much out on the return to tdc most of it said goodbye to the cylinder at bbdc,this is where all the exhaust manifold tuning come's into play and during overlap to scavenge some fresh charge in using the exhaust gas exit speed,its very complex but good luck in your stuidies,wait till you get to the most complex part of a ic engine the camshaft.
andys2 said:
Whats the video your watching and where did you get it from?
Andy
Andy,sorry i must keep with the beat,totaly missed this,the video i am watching is for a 4 cam Lotus designed V8 called the LT5(could that be a clever abreviation of LoTu5
),i got it from a guy in the states off a ZR-1 corvette forum and the video takes you through all the steps of engine maintenance from crank overhaul to cam timing. Cliff
>> Edited by ZR1cliff on Sunday 4th September 17:14
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff