does a rich mixture mean air mass is increased aswell ?
Discussion
All mainstream cars, whether high or low performance, vary the mixture to suit the engine load in various ways. In modern cars via mapped injection systems, in older cars via various different jets in the carbs, and things like accelerator pumps to cope with rapid throttle openings.
Too lean a mixture from the best-power setting for a given engine reduces power, all the way to zero if carried too far. So does too rich a mixture.
Mixture is measured by the ratio of air mass to fuel mass, so your question about air mass does not make sense.
Too lean a mixture from the best-power setting for a given engine reduces power, all the way to zero if carried too far. So does too rich a mixture.
Mixture is measured by the ratio of air mass to fuel mass, so your question about air mass does not make sense.
To answer the title: No. 
A rich mixture means there is more fuel in the air-fuel mixture than can be burnt completely by the air present; a lean mixture means that there is more than enough air present in the mixture to burn the fuel present. Both conditions have implications - either exhaust emissions or damage to the engine (or even both).
Modern petrol cars run around a 'stoichiometric mixture' (to entertain the exhaust catalyst) meaning there is enough air to burn the fuel present - in the most simplistic terms, anyway
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Diesel engines usually run 'lean', as they don't have a throttle.
I suspect the OP is confusing 'rich mixture' with turbocharging - in which case he is right
, more fuel plus more air still around stoichiometric mixture (for the modern stuff with cats and in simplistic terms, anyway )...
A rich mixture means there is more fuel in the air-fuel mixture than can be burnt completely by the air present; a lean mixture means that there is more than enough air present in the mixture to burn the fuel present. Both conditions have implications - either exhaust emissions or damage to the engine (or even both).
Modern petrol cars run around a 'stoichiometric mixture' (to entertain the exhaust catalyst) meaning there is enough air to burn the fuel present - in the most simplistic terms, anyway
.Diesel engines usually run 'lean', as they don't have a throttle.
I suspect the OP is confusing 'rich mixture' with turbocharging - in which case he is right
, more fuel plus more air still around stoichiometric mixture (for the modern stuff with cats and in simplistic terms, anyway )...Two separate things.
Fuel mass - fuel.
Air mass - air.
Too much of either and it causes problems, not enough of either and it causes problems, Too much of both and things start breaking.
Too much fuel - combustion doesn't work properly as there is too much fuel to be combusted, which means some fuel is un-burnt, some is partially burnt... Worse fuel consumption, much worse emissions, lower power.
Too much air - pretty much the same, as yet again, combustion doesn't work properly.
Air mass is pretty much constant, leaving aside effects of pressurising air prior to combustion (ram air, supercharging and turbo-charging are the main ones, but there are other ways of doing it) leaving fuel flow rates, which can be altered - carb-fed engine's by simply putting more in by supporting jets, injection engines by more complex processes thanks to additional sensors and electronics (why fuel injection is more fuel efficient in most cases)
running rich to create more power works to a point, but too far and it causes the too much fuel scenario.
Fuel mass - fuel.
Air mass - air.
Too much of either and it causes problems, not enough of either and it causes problems, Too much of both and things start breaking.
Too much fuel - combustion doesn't work properly as there is too much fuel to be combusted, which means some fuel is un-burnt, some is partially burnt... Worse fuel consumption, much worse emissions, lower power.
Too much air - pretty much the same, as yet again, combustion doesn't work properly.
Air mass is pretty much constant, leaving aside effects of pressurising air prior to combustion (ram air, supercharging and turbo-charging are the main ones, but there are other ways of doing it) leaving fuel flow rates, which can be altered - carb-fed engine's by simply putting more in by supporting jets, injection engines by more complex processes thanks to additional sensors and electronics (why fuel injection is more fuel efficient in most cases)
running rich to create more power works to a point, but too far and it causes the too much fuel scenario.
The stoichiometric ratio for burning petrol is 15:1 and the cars air mass meter will adjust the volume of air to maintain the same ratio throughout,except under load or acceeration where the mixture is richer, however if the op is referring to not rich mixture but higher octane fuel in a performance car the volume of air will be adjusted to suit and may be lowered.
Edited by ronaldo342 on Thursday 20th February 23:02
ronaldo342 said:
The stoichiometric ratio for burning petrol is 15:1 and the cars air mass meter will adjust the volume of air to maintain the same ratio throughout, however if the op is referring to not rich mixture but higher octane fuel in a performance car the volume of air will be adjusted to suit and may be lowered.
It will also change the ignition timing to suit the higher octane rating (assuming the dark arts electronics have this capability) ...Backdrop to all this is maintaining the stoichiometric ratio, which is 14.7:1 air/fuel ratio by mass - but the engine will then run slightly richer than this to allow any remaining CO to be oxidised to CO2 and (in simple terms) any unburnt hydrocarbons converted to water and a bit more CO2 in the catalytic converter under normal running conditions...
Your engine is just a mechanical air pump, its pretty much a fixed amount of air dependent on temp/altitude/revs & there is no changing the airflow beyond the throttle plate. The engine management will just measure the airflow & it simply adapts the fuel supplied through the injectors to adjust the rich-lean air fuel ratio.
ronaldo342 said:
The stoichiometric ratio for burning petrol is 15:1 and the cars air mass meter will adjust the volume of air to maintain the same ratio throughout,except under load or acceeration where the mixture is richer, however if the op is referring to not rich mixture but higher octane fuel in a performance car the volume of air will be adjusted to suit and may be lowered.
It works the other way round, the throttle adjusts the volume of air, then the ECU adjusts the amount of fuel injected to match the readings.maverickdrives said:
so basically when you get a backfire, it means the fuel is unburnt and there is more fuel than the air required to complete combustion? I have seen that happen alot on sti's
Most turbo cars richen up as the boost gets upped to try and protect the engine (less change of det under boost). My old MR2 used to throw fireballs when running high boost.maverickdrives said:
so basically when you get a backfire, it means the fuel is unburnt and there is more fuel than the air required to complete combustion? I have seen that happen alot on sti's
Yes and no, it may be that there is not enough air. But it's equally likely that there was incomplete mixing of air and fuel in the cylinder so it didn't all react, then it burns in the exhaust when they do mix.aw51 121565 said:
ronaldo342 said:
The stoichiometric ratio for burning petrol is 15:1 and the cars air mass meter will adjust the volume of air to maintain the same ratio throughout, however if the op is referring to not rich mixture but higher octane fuel in a performance car the volume of air will be adjusted to suit and may be lowered.
It will also change the ignition timing to suit the higher octane rating (assuming the dark arts electronics have this capability) ...Backdrop to all this is maintaining the stoichiometric ratio, which is 14.7:1 air/fuel ratio by mass - but the engine will then run slightly richer than this to allow any remaining CO to be oxidised to CO2 and (in simple terms) any unburnt hydrocarbons converted to water and a bit more CO2 in the catalytic converter under normal running conditions...
maverickdrives said:
so basically when you get a backfire, it means the fuel is unburnt and there is more fuel than the air required to complete combustion? I have seen that happen alot on sti's
Lean mixtures can also cause a backfire/popping from the exhaust. The mixture is so weak that it misses the spark plug and it ignites when it goes into the exhaust manifold.Backfiring and popping aren't the same thing. Backfiring was more a carburettor thing, and the flame literally travelled backwards up the inlet port.
As above, the engine is just an air pump, and it pumps whatever mass of air suits for an RPM and throttle butterfly position. The mixture is controlled by varying fuel.
If a rich mixture also meant greater air mass, then the mixture wouldn't get richer but the engine would become powerful - which obviously can't happen. You don't get a bigger engine from just firing more fuel in.
As above, the engine is just an air pump, and it pumps whatever mass of air suits for an RPM and throttle butterfly position. The mixture is controlled by varying fuel.
If a rich mixture also meant greater air mass, then the mixture wouldn't get richer but the engine would become powerful - which obviously can't happen. You don't get a bigger engine from just firing more fuel in.
MegaMinion said:
Modern engines don't try an maintain stoichiometric air fuel ratios, because it's physically impossible to get stoichiometric conditions. They run lean not rich as running rich is what causes CO, you would run rich to oxidise NOx.
They alternate between lean and rich all the time in closed loop, if they use a narrowband sensor (most cars).Gassing Station | General Gassing | Top of Page | What's New | My Stuff