prob 2.11

prob 2.11 - actually entails that the lean running engine...

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Question 2.11 A/F effect on power output, efficiency and emissions for: a) constant speed b) constant throttle c) constant ignition timing Answer: At constant speed the power output maximizes at ~ 12:1 A/F and drops off either side. The efficiency maximizes at A/F~ 16.5 :1 and drops off either side of this. Emissions of NOx peak near the equivalence ratio 0.9 (~16:1 A/F) and drop either side. HC output is also minimum near this AF ratio. CO rises rapidly with mixtures rich of stoichiometric and essentially disappears when running lean. See figures 2.9, 2;10, 2.11. There is little difference in the response to fundamental behavior at a constant throttle setting, other than then the fact that the engine will speed up with a richer mixture and slow with a leaner one (within limits) as the power output varies due to the fuel mixture change. The constant speed requirement in the above part of the question
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Unformatted text preview: actually entails that the lean running engine needs to see more throttle opening for a given amount of power thus reducing the pumping losses slightly. The opposite is true for rich operation. The above trends of best efficiency and power with an optimal lean A/F setting, and maximum power for a particular rich setting, will not be as pronounced if the ignition timing is not varied to suit the flame speed. Lean mixtures (slow flame speed) will need more timing advance to put the peak pressure at a point in the cycle where it does the most work. The emissions characteristics with fuel mixture changes will, in the instance of going lean without extra timing advance, decrease NOx however the same late start of combustion will also put more HC and CO out the exhaust because the combustion process is still incomplete....
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