Oh oh, this could take a while. I'll try to be brief. In truth, the answer
to your question is quite simple. The number one piston is at TDC when the
TDC mark on the crank pulley lines up with the pointer. So is the number
four piston. And 180 degrees of crankshaft revolution will put number two
and three at TDC.
That is not the whole story, however, with an Otto cycle motor. The short
synopsis of the Otto cycle is suck, squeeze, bang and blow. And no, I'm
not talking about what my wife and I ... uh, nevermind.
Suck: intake stroke of the piston - draws in the fuel and air mixture
Squeeze: compression stroke - compress the mix before setting it on fire
Bang: power stroke - spark plug ignites the mixture, kaboom!
Blow: exhaust stroke - push out the burned gases
So on a motor like the Spitfire, and every other four cycle internal combustion
engine on the face of the planet [Wankels and two strokes are different] the
piston in any given cylinder is at top dead center twice during this process.
It hits TDC at the point where the compression stroke [squeeze] finishes and
the power stroke [bang] begins. And it also hits TDC at the point where the
exhaust stroke [blow] finishes pushing out the burned gases from the last
cycle and the intake stroke [suck] starts drawing in a fresh mix. For the
purposes of this discussion, assume there is no overlap in cam or ignition
timing, we'll keep it simple for the moment.
To run a leakdown test, I suggest you remove the rocker cover and all the
spark plugs. An assistant will speed the process, but is not required.
Doing the test on an engine at operating temperature is a bit more accurate,
but you can usually get the same picture with slighty different numbers from
a cold, more finger friendly engine.
With the rocker cover off rotate the motor until the two valves on the number
four cylinder, the rearmost one, are 'on the rock.' That is, the exhaust
valve is closing, ending the 'blow' part of the cycle, and the intake is just
beginning to open for the 'suck' cycle. With the valves on #4 at this point,
the number 1 cylinder is at the point between 'squeeze' and 'bang.' Both
valves are closed and the piston is at TDC. Screw the leakdown fitting into
this cylinder.
Now, here's whare an assistant could come in handy. With the tester in place,
one could easily apply the air pressure and get a reading, except that with
the 100 psi or so of applied air pressure the piston will be pushed down in the
bore, rotating the crank and opening a valve a bit, giving you a completely
useless reading. If the car is in gear at the time, some festive movement
will also occur.
So the trick is to have someone in the driver's seat standing on the brake
pedal with the gearbox in fourth, keeping the crank from moving. I have a
long hose on my leakdown tester with a valve in it, so I can be sitting in the
seat, pushing the brake and then crack open the valve to apply the pressure
to the tester and get the reading. Relieve the pressure on number one, rotate
the crank 180 degrees and test the next cylinder in the firing order. Repeat
process until all four are done.
In truth, it can be informative to get leakdown numbers when the piston is at
TDC, the middle of the bore, and at the bottom. This can give you an idea of
what condition the cylinder walls are in, but is more work, as you need to
remove the rocker assembly. Chances are you'll find out what you need to
know with the basic TDC procedure.
Anyone still awake?
mjb.
/// spitfires@autox.team.net mailing list
/// To unsubscribe send a plain text message to majordomo@autox.team.net
/// with nothing in it but
///
/// unsubscribe spitfires
///
|