Technical Question

Can you recommend a souce for information on the combustion process as related to the automobile engine? Specifically, why is an engine more likely to misfire when under a light load? I need the technical answer. Any suggestions would be appreciated.

Thanks, Greg.

Oh. You want the simple stuff? On your Google bar, put ‘physics of internal combustion engine’. Those sites should have enough information.

Be advised there may not be a simple answer to your question since the misfiring under a light load could be any one of a number of things.

Having this problem on a particular car?

It sounds like he has a class project…


A properlu operating engine is not more likely to misfire under a light load. It sounds to me like when you release the throttle the engine is temporarily being slightly mismetered.

An engine is an air pump. Every time the piston goes down on the intake stroke it pumps in air. Every time it goes up on the exhaust stroke it pumps out expended exhaust, combustion byproducts. The total volume inhaled and exhaled is simply the volume of each cylinder multiplied by the number of cylinders.

If your engine is hard-wired to the wheels, like with a manual tranny, then when you let off the accelerator pedal without pushing the clutch pedal in, the engine continues to try to inhale the same volume it did when the throttle plate was wide open. Only now you have a restriction…a throttle plate blocking the windpipe. Airflow pre-throttle goes down because less air is being drawn in. Vacuum post-throttle spikes because of teh restriction in the air intake. In a properly functioning fuel injected engine, the position of the throttle plate is measured by the throttle position sensor, airflow in is read by the mass airflow sensor, the vacuum post-throttle is measured by the manifold absolute pressure sensor, engine speed is measured by the crank or cam speed sensor, and the ECU runs these readings through a computer program to meter the fuel being supplied by the injector(s) (by adjusting the injector pulsewidth) such that the engine decelerates but has sufficient fuel in the mix to continue to run smoothly. The output of these processes is monitored by the upstream (pre-cat converter) oxygen sensor and tweaks made to the adjustment.

Under constant light load, these same systems should be reading the sensors (along with the engine temp) and adjusting the pulsewidth of the injector accordingly such that the engine has the proper metering (mix) to continue to run smoothly.

Your engine has a boo-boo somewhere in these systems. Could be a sensor, could be a malfunctioning injector, could be a whatever. Tell us more. We need more information about teh vehicle and more descriptive narrative to go on.

Never let it be said that I declined to help a student with a class project. I won’t give him the answer, but hopefully I’ll help him learn how to figure out the answer.

Glad you asked. I’ve worked on cars almost my whole life. Some twenty years professionally. This is not particular to any car. The question I have pertains to all gasoline piston engines. Looking for answers like this is extremely time consuming using something like Google. Someone knowlegeable on the subject could save ton of time. hellowkit is right, this is not simple question. If you care to share what you know I’d be very happy to listen.

The best way to clarify my question is to consider what is happening when you ask your car to accelerate moderately as you would when you want to speed up from 25mph to 35mph. You press the throttle pedal slightly the manifold pressure drops slightly. For the last 50 years this “tip-in” as it’s called is the time most likely to see a “lean misfire” or stumble. It’s also the most likely time to hear ping, not at WOT. An engine is more likely to “ping” with this slight change in manifold vacuum than when you “floor it”. Again, this applies to all gas engines. I’m looking for technical sources. Thanks for the feedback. My daughter used to ask questions about various things. I would have to tell her, “I can answer your question; but, you don’t have the background knowledge to understand the answer.” In a way, that’s the way I feel about your question. To help to fill in your knowledge base (and, as a refresher) go to the above cited website. Also, at the bottom of the article, click on “Physics in an Automotive Engine” for some more technical stuff.// You could learn more about engine management by going to sites like:, and,

“For the last 50 years this “tip-in” as it’s called is the time most likely to see a “lean misfire” or stumble. It’s also the most likely time to hear ping, not at WOT. An engine is more likely to “ping” with this slight change in manifold vacuum than when you “floor it”. Again, this applies to all gas engines. I’m looking for technical sources. Thanks for the feedback.”

I disagree with your premise. Misfire is MOST likely to occur under heavy load in MOST gasoline engines. The misfire YOU describe is most likely caused by a malfunctioning EGR valve or dirty injector, not in a properly operating engine…

Aha! It was a trick question! A test!

Pinging during this “drop in” isn’t caused by a drop in manifold pressure. It’s caused by the momentary leaning leaning out of the mix. Suddenly a rush of air goes down the intake when the restriction of the throttle plate is removed and it takes a tad for the injectors to respond to the TPS signal and open the injector for longer pulsewidths. More oxygen is like using a bellows in a fireplace; the burn gets suddenly hotter. The hot in the chamber causes a secondary wavefront to form from compression and residual heat and clash with the primary wavefront when the heat energy is added via the spark. Modern engines have “knock sensors” which are sensors in the head that detect the shock wave from pinging (pinging is actually the sound teh shockwave makes) and retard the spark slightly such that that process happens a bit later in the cycle and the secondary wavefront is eliminated.

Modern engines respond in extremely small timeframes. And they have a Exhaust Gas Recirculating (EGR) system to add a bit of inert exhaust gas to the mix to displace some incomng air (oxygen) and prevent pinging. Pinging should not occur in a properly functioing engine when the proper octane gas is used. Many modern engines boost intake pressure using turbochargers or superchargers, and that added compression causes higher heat in the cylinder and leads to too quick a detonation of the mix. Higher octanes don’t detonate as quickly.

But, I’m not used to hearing pinging called “misfiring”. I usually interpret "misfiring’ as meaning the lack of detonation. So I misunderstood the question completely. Semantics.

I can recommend a few good books:

Auto Fundamentals
By Stockel, Stockel and Johanson
Published by Goodheart - Wilcox
ISBN 1-56637-138-4

Automotive Engines
By Crouse and Anglin
Published by MacMillan/McGraw Hill, (800) 338-3987
ISBN 0-02-801099-X

Complete Engine Performance and Diagnostics
By Robert Scharff & Editors of Motor Service
Published by Delmar Publishing / Thompson learning, (800) 477-3682
ISBN 0-8273-3579-2

but beyond that I have no sources that I can think of to reference. I’m useless.


gregs25: TSMB had it right on the money, but I’ll paraphrase. (By the way, are you an auto tech instructer, TSMB?) During light accell as opposed to TROMP, mix is leaner, burns hotter- consider an oxy- acetalene torch: just acetalene- it burns hot. Hit oxygen; it burns much hotter. Lean mixture takes longer to burn, too: imagine gunpowder laying on a table top. W/ grains close together, if you toss a match in the

middle of table, match will ignite particles near match; the burning grains in turn ignite their neighbor; combustion (flame front) moves outward from middle in a circle. Put fewer grains on table spread out evenly, toss match; flame front moves outward more slowly; grains are further apart; takes longer for a grain to ignite its neighbor. Now go inside combustion chamber- leaner mix not only takes longer to burn,

but burns hotter (oxy- acetalene torch example). Spark ignites charge in the middle (think of table top); combustion takes longer; is hotter than a rich mix. (again- oxy acetalene example) If something’s not right w/ fuel management or ig. system- (God, it could be a million things- combustion chamber carbon deposits; making chamber smaller, more pressure (more heat). Too low octane fuel- now chamber’s too hot relatively speaking; for lower octane

fuel.)- something’s not right: spark ignites charge in middle, mix takes long to burn;, before normal outward burning can ignite the whole charge, fuel bubbles out at edge of chamber are under greatest pressure (think of a crush at a soccer match- people up against fence are under greater pressure than those further in from fence) pressure means heat- these bubbles ignite by themselves; move IN in a flame front; the 2 flame fronts collide (ping).

Alright, that’s ping; now misfire which no one seemed to addresss too much: cruising at 25 ; accel lightly- you get lean mix (compared to TROMP). Spark jumps air gap, but it’s not an air gap, is it? It’s an air-fuel gap. Leaner the mix, more voltage needed to jump gap; (untill pure air would have infinite resistance.) Marginal ig. coil. or more likely, plug wires w/ too high resistance, might dis-allow enough voltage

to jump gap. TSMB- help me out here-does greater pressure in chamber require greater voltage? I think it does. Anyway, my trade school instructor told us light acell was a good PRELIMINARY check for bad plug wires; so did my first shop foreman who had worked for my instructor. gregs25: This is just one example of a cause for miss under light acell, but one of the most common ones. Hope this has helped. KS