Subject: Re: Hard Starting Eldo
Author: Patrick Hayes
Date: Apr 14, 2005, 4:07 PM
Post ID: 1718704089
Lannis wrote:
| But if the engine is turning over, and not firing, then the ignition timing can't possibly have anything to do with the slow turning. Can it? Someone set me straight if I'm off base.... |
OK, Lannis, you're off base. The advanced timing does not necessarily
cause a great CHUFF and whack the motor into reverse motion. Understand
that compressed mixture of gasoline and air does not "explode" (which
would be interpreted as instantaneous). The mixture simply burns. Very
rapidly, starting at the spark plug tip and working its way outward like
ripples in a pond. There is a definite and measurable thrust wave as
the flame front progresses. The result of the flame front is to expand
the volume of gaseous byproduct, increase the total volume and push the
piston down. If timed correctly, the piston goes over the top and
starts downward as the burning process occurs.
If you get the spark started waaaaay too early, the burn cycle starts
while the piston is not anywhere near the top and the resultant
expansion of gas will force the piston back down, backwards. Very rare
as this requires a LOT of advance timing and a hot motor.
If you get the spark started just a little too early, the momentum of
the flywheel carries the piston over the top of its stroke a the same
time that some of the fuel/air mixture is starting to burn. This can
seriously fight back against the starter motor rotational power. It is
characteristic of the symptom our Guzzi user describes.
BTW, read about the flame front above. Imagine this expanding ring of
flame. Behind it is exhaust byproduct. In front of it is unburned fuel
and air. The process of combustion at the flame front causes a huge
expansion of gaseous volume behind it. This expansion helps to push the
flame front forward or outward faster than it would move on its own
accord. Let's say your engine normally measures 9:1 compression.
That's the pressure that the mixture is under at the start of the flame,
instant of spark, at the plug tip location. But the burning and exhaust
production continue to push a wave forward and the final few cc's of
unburned mixture out at the cylinder edges get compressed by this wave
before the flame arrives and might actually experience something like
12:1 compression ration before they burn. If you get this system a
little too hot, or a little too dirty, then the unburned portion in
front of the flame can actually over-compress and ignite completely and
spontaneously, regardless of the location of the flame. This is diesel
explosion and this is exactly what you hear when your engine pings on a
hot day under load. A certain Mr. Diesel learned these things by
embedding a glass window in a cylinder head and looking to see what
happens inside.
BTW #2. The reason we buy premium gasoline for our high compression
engines is NOT that it produces more power, but that it actually burns
with a slower flame progress rate at normal temperature and pressure
ratings. Regular fuel would spontaneously explode, ping, diesel. So we
use slower burning fuel to give the piston a chance to get over the top
and start downward as the flame progresses.
That's my story and I'm sticking to it!
Patrick Hayes
Fremont CA