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Bruce Sutherland
United Kingdom
1564 Posts |
 Posted - 09/06/2008 : 00:39:33
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George et al,
"What does stoichiometic mean?"
I am not a combustion expert, but a summary of a text book description follows.
For spark ignition engines (i.e. petrol as apposed to diesel) the ideal theoretical quantity of air to fuel for complete combustion is at a mass ratio of 14.7:1. This ratio value is also termed 'stoichiometric'.
Accurate control of the air:fuel ratio to stoichiometric is required for maximum efficiency of catalytic converter systems and fine tuning of the fuelling is achieved using a lambda sensor which senses an excess or deficiency of oxygen in the exhaust (excess = lean & visa-versa). This provides a closed-loop feed-back control of the fuelling.
Stoichiometric is defined as lambda=1 (air:fuel (A/F) 14.7:1).
However maximum power is generally achieved at a slightly richer fuelling (air deficiency) at lambda= 0.85-0.95; but richer fuelling causes the tailpipe CO to rise (more emissions).
I contrast, best/lowest fuel consumption is achieved with an air excess (lean) at lambda=1.1-1.2. This is for engines having air intake manifold fuel admission (i.e. 'conventional' fuel injection and carburettor fuelling systems).
In modern vehicles the very narrow limits required for A/F mixture control to simultaneously achieve low fuel consumption, power output, and emissions mean that multi-point (usually one fuel injector for each cylinder) electronic digitally controlled fuel injection systems have taken over from the coarser/cruder/approximate fuelling provided by the analogue control from a carburettor system.
A modern Engine Management System (EMS) has many sensor inputs - (typically air mass flow, ambient temperature and pressure, coolant temperature, engine speed, road speed, throttle opening, rate of throttle opening, fuel temperature, æknockÆ (detonation) sensor, Lambda exhaust oxygen sensor, etc - I must have missed a few) - these sensor inputs enable ever more precise control of both fuelling and ignition timing which achieves good starting, driveability, performance, economy, and low emissions under all operating conditions.
In view of the above, the limitations of simpler carburettors and ignition systems fitted to earlier vehicles (e.g.Triple-M) become apparent, but their basic function remains the same which is to maintain close to or slightly richer than A/F 14.7:1 under all operating conditions.
There are text books with fuller explanations and there will be other members of this forum better versed than I in this subject who will correct my mistakes.
A æstarter for tenÆ. Hope this helps.
Bruce. |
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