Why can't an internal combustion engine self-start from zero rpm without being spun-up by a starter? 6

[LMF5000]

Land vehicles that use external combustion engines (like steam locomotives and steam cars) don't have a clutch and don't idle. They simply stop the engine while still geared to the drivetrain. To restart they simply open the steam valve and the engine spins up the load from zero rpm.

Thinking of ancient petrol engines, I can see why some non-zero running speed is required for the engine to start running. The spark plug is fired by a magneto that needs a minimum flywheel rpm to make a usable spark; the carburettor admits fuel via a venturi that needs some airflow to actually pump fuel into the cylinder.

But what's preventing a modern engine from just starting at zero? With a 4-cylinder engine there's a 1-in-4 chance of one of the cylinders being somewhere in its power stroke at any one time. The crank position sensor will tell the ECU which cylinder it is. The fuel injectors and fuel pump can fill that cylinder with a combustible mixture. The battery and ignition coils can produce a spark even though the engine isn't spinning. Igniting that mixture should create enough pressure to get the engine spinning, at which point the other cylinders pick up and the engine accelerates to idle rpm.

Any thoughts?

 

[gruntguru]

The key aspect being a direct injection system (vs manifold or port injection) and the ability to control or sense that a piston happened to end up someplace just past TDC on the power stroke.

The sensing aspect is trivial - many engines already have that capability. Same for DI, although there is significant additional cost associated with DI vs PFI.

je suis charlie

 

[GregLocock]

OP - no. With no compression the torque available from a zero rpm start is not enough to move the car. You could supply a blast of compressed air as well, and then burn in that. I think that was used on some aircraft.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Pancholin]

Normally a four stroke engine uses CARBURATION, COMPRESSION, , IGNITION, EXPANSION. Compression is missing and any fuel will condense.

 

[gruntguru]

CARBURATION - whats that?
Fuel condenses - depends on the temperature. Fuel vapour in a hot engine after shut down will remain vapour for a long time.
I have made engines run just sniffing the vapour from the top of a fuel can. (The engine did the sniffing - not me)

je suis charlie

 

[CWB1]

my main line of thinking was whether we could eliminate the clutch/torque converter and make internal-combustion engines drive the wheels from zero rpm like steam-engined cars did, and like electric cars do currently.

Sure, give me enough gears and I'll move the world. In the early days it wasn't uncommon for forgetful or lazy operators to have a diesel engine restart itself shortly after shutdown due to heat soak, leaky injectors, and other factors, many had manual air and fuel shutoffs added partially for this reason. When left in a low gear, many folks found their equipment would relocate itself.

 

[WGJ]

As Waross said about the Model T, a similar thing could be done with 1930s Rolls-Royce models.

https://www.motorsportmagazine.com/archive/article/may-1975/48/starting-rolls-royce

Bill

 

[BrianGar]

I thought I had imagined something some what related to this thread so I had to double check - My brain hadn't made it up,

http://www.6x6world.com/forums/engines/16141-2-stroke-reverse-snowmobile-engine.html

''actually all engines fire while still on the up stroke about 6 to 10 degrees BTDC (before top dead center)
in the snowmobiles it is a computor controled process where it stops the fireing till it sences that the RPM are correct and it is nearly stoped then it fires about 20 degrees BTDC thus being too early it will push the piston down before it reaches TDC and cause it to start turning the wrong way then it immediatly fires at the normal 6 - 10 degrees again but in the oposite direction.
You can do this by accident when you accidently hit the kill switch on a snowmobile and just by luck pull it on at the right time, i think this is how it was invented (note; if it happened to you by accident you instantly realized something was wrong because it ran like crap and nothing happened when you hit the throttle, all you had to do was shut it off and re-start it and all was well) ''

Brian,

 

[enginesrus]

Some here seem to think compression pressure is needed to initiate the start that the OP is talking about.
Not so, just look up on youtube about using ether to seat a truck tire to a rim. There would be enough pressure created to start the process.

 

[gruntguru]

I watched a guy welding a car fuel tank once (I kept my distance). The occasional whooomph of flaming gases exiting the filler neck (yes he was smart enough to take the cap off first) was something to behold.

je suis charlie