What causes an engine to backfire?

Factual Questions
1

This has always puzzled me: What causes an engine to backfire? Does fuel drip into the hot exhaust line and combust in the exhaust line?

2

If you have fuel dripping into your exhaust line you don’t have backfires, you have an afterburner… :slight_smile:

The fuel-air mixture in the cylinders is supposed to be stoichimetric balance, that is there’s just enough of each to complete combustion. For one reason or another you get a rich mixture (too much fuel) and the unburned portion flows down the exhaust stream. The exhaust manifold is sufficiently hot, usually, to ignite that fuel once it finds some air to mix with.

My brother’s van used to do the best backfires. He’d just turn the ignition off for a few seconds going down a hill and then, when he’d turn it back on, KABOOM. Loads of laughs for teens…

3

Usually it is a result of wrong ignition timing. In exceedingly rare cases it could be bad valve timing.

4

Basically, that’s it. Either a cylinder does not fire, or the air and fuel do not completely burn in the cylinder. The unburned fuel ignites when it reaches the hot exhaust.

To be picky though, a backfire happens when the air and fuel mixture ignites in the intake; the fires goes backwards through the intake manifold, carburetor or throttle body and air filter. A pop in the exhaust is called a misfire.

5

Oops. I should have read your post more carefully. See the answer by MonkeyMensch

6

a) I understand what you’re saying about air-fuel mixtures, but I don’t totally follow the difference between “exhaust stream” and “exhaust line”. Overall, I guess you’re saying backfire occurs in the exhaust manifold.

b) What if a riding mower backfires? We heard a loud, sharp “POP!” when shutting off the engine (of a brand new mower used for just the second time.) Should we be concerned? Thanks, - Jinx

7

OK, so out of curiosity: What causes misfire? If a cylinder fires prematurely, you get knocking in the engine (cylinder) correct? I’ve heard this called misfiring when describing the timing of the engine, overall. Such as “the engine is misfiring”, or " a piston (or cylinder) is misfiring.

Is a pop in the exhaust caused by excess fuel combusting in the exhaust manifold…or, exhaust line?

Thanks for explanations,

  • Jinx
8

Actually, a pop in the exhaust is called afterfire. Usually, it’s whatever air it finds in the exhaust that makes it ignite, not the exhaust heat itself.

Backfiring can be caused by having an excessively lean mixture that may burn slowly enough that combustion continues through the power and exhaust strokes. When the intake valve opens, it can ignite the contents of the induction system/intake manifold.

9

By “exhaust stream” I was referring to the manifold and exhaust pipe as a functional unit. The exhaust manifold is really hot. Look at a picture of an engine being tested at full load on a brake. The manifolds will be close to orange in color with heat being transferred. I figure you don’t run your mower that hard but they are still hot.

I wouldn’t be concerned. Excited, but not concerned. What’s probably happening is that the engine turns over a few more times from inertia after the ignition is turned off. That action pulls fuel-air mixture through the engine, now unburned with a lack of spark, and it chances to ignite in the exhaust system.