Quote:
Originally Posted by robchalmers
Big Big theory
On the forum there seems to be two schools of though of how to improve economy.
Firstly what I’m calling hot camp. The concept as I grasp it is that warmer air is less dense so the is less 02 hitting the lambda sensor in the exhaust – ECU thinks the car is running rich and therefore drops the injected amount. Thus less fuel used. This also goes with the idea of choking the intake to reduce the amount air that can get into the engine for the similar effect. I’m also putting into this group the concept of lower Octane fuel = more economy, that one still puzzles me
Second smaller group is what I call the big bang theory group. I was always told to get the most efficiency you need to get the most from the least. This goes off the CAI theory of more dense air produces a bigger bang per unit fuel, yes the ecu will see more air burnt and resultantly increase the mixture, but the extra work done means you can back off the throttle thus keeping the same mixture ration control by the ECU but just achieve the same work on less fuel. I’m also combining this with the higher octane side too – as to me it relates directly to the calorific/energy out value of fuel. To my mind if your ecu in the car keeps the mixture the same/burnt rate/ timing self optimised as per an modern ecu, for every ml of fuel a (Hi oct) you would generate more energy, per bang than fuel b (lo oct) and as a result you’d require more fuel to provide the same energy needed to cruise at a given speed.
If for example we take a pretty common direct stratified inject petrol engine that doesn’t require the fuel mix to vaporise in between an old school carb and piston would have though the best thing is to have cold dense air to create the most complete bang from a tiny amount of a hi octane fuel?
I’ll grant you that I might be missing something please set me straight
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I'm kinda in the middle as far as the
big bang people and the
hot camp.
I think the
hot air people are seeing more improvements from fuel vaporizing when ambient temperatures are low. The fuel is having a hard time going from fuel droplet to fuel vapor. Plus with today's fuels, they the "oil companies" have been told to reduce VOC from big brother. So the fuel has some issues when it comes to vaporizing. You can see this when you accidentally spill some fuel on the ground and watch it evaporate. It takes for ever compared to fuel 30 + years ago. On stock ecu's they will also notice as you said a warmer reading from the IAT sensor and will reduce the injectors pulse width.
I agree with the
big bang people also. This comes down to improving BSFC. Making the engine utilize every amount of BTU's it can from the fuel. There is some room to improve from a stock engine. But from a emission standpoint it might not be good.
This is where I'm at now. I have gone from a 14.7 A/F ratio to a 15.5 A/F ratio to now a 17.1 A/F ratio. I haven't fuel-logged my 17.1 A/F ratio yet but as far as MPG it will be a success.( still working on timing values)
This is a advantage I have over a lot of people on this forum. I can actually see in real time where my injector pulse width is coming from on my fuel maps and ignition maps. My efi system high lights the cell that your running in.
What I have found that's interesting is that when you just start taking away fuel you will lose power and you will notice that it will go to the next higher load cell. Plus you can see it on the tps log. The way I over come this and get back to the same injector pulse width and fuel cell is to increase timing at that cell by a few degrees.
At these lean A/F ratios the car needs more timing because a leaner A/F has a slower burn rate then a richer one. At least at this A/F level. Now when the car is at full load this will not apply because the fuel is being used as a thermal cooling control for combustion. The extra fuel is just going out the exhaust but if its not their you will run into a knock situation.