CamaroUK

Why does a low idle speed produce more exhaust hydrocarbons than compared with a fast idle?

I would have assumed that the air/fuel mixture would have more time to fully combust at a low idle and therefore putting out less HC, but apparently not. This effect also seems apparent in cammed cars.

How does this also effect the amount of Carbon Monoxide emissions, a low idle produces a high percentage of CO but a fast idle produces a negligable amount?

Anyone know why?

CamaroUK

Anyone?

Come on, someone must know!

LiENUS

Valve overlap at the low idle you're getting unburnt air almost right into your exhaust.

CamaroUK

Would the faster intake/exhaust port velocity of a faster engine speed not exaggerate this effect?

I would have thought intake/exhaust scavenging would be virtually non existent at idle because of the low speed of the exhaust gas.

Assuming you are right, what difference does engine speed have on scavenging?

flatlander757

I'd imagine it is directly proportional because you don't need scavenging at lower rpms and the pulses only move as fast as the engine is putting them out.

CamaroUK

This is what I can't get my head around, the exhaust pulses are what create the low pressure in the chamber (According to my David Vizard book) and pull the intake charge through. I can understand this being a big influence at high rpms, but as you said, the slower the exhaust pulses such as at idle, the less effectiveness they should have on scavenging (or the same if it is proportional) and pull the same or less intake charge though into the exhaust port.

This can't be the case though as the gereral results disagree with me.

flatlander757

Well if anything I'd think it mainly reads higher because the valves are open longer than at higher rpms. So since it is able to draw in more air at lower rpms than compared to high rpms, the O2 sensors should tell the PCM to dump more fuel in.

I dunno, Thank God my 94 doesn't need smogged