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I believe that the whole point of the ECU is to achieve complete fuel consumption. That means that the petrol to air mixture needs to be just right. Now that would have been simple if it weren't for varying fuel quality and/or composition.
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That's a Bingo. The main aim of Electronic control is to reduce emissions and make the world a greener place!
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Not exactly. O2 sensor is basically for feedback. The amount of injection depends on air pressure and temperature in the intake manifold.
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As Oxyzen pointed out, the O2 sensor(s) are placed in the exhaust manifold and before &\ after the catalyst. Its only for feedback if fuel injected is completely burnt or not. Fuel injection depends on the amount of air that has come in (For petrol cars) through the air filter.
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1. Does compression ratio have any effect on this?
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Yes! Higher the compression ratio, more efficiency & more power can be produced. However, not all engines can sustain high compression ratios as high amounts of heat is produced and also petrol can knock. Thus, the CR is limited to around 10-12.
Knocking also happens depending on fuel composition. Thus, If a car can sense the knocking characteristics of the fuel, it can take some corrective action. Indian cars like Ritz, Swift, Fiesta etc have knock sensors. You can find out if you get slightly better mileage while using higher quality petrol. This is among other reasons, due to the knock sensor.
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I'll just post a series of theoretical scenarios.
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Before the scenarios, lets get a few things clear.
- more throttle opening, better efficiency since the engine doesn't have to do work drawing air from a not-so-open throttle. Remember, a throttle is called so because it restricts air flow into the cylinders! Imagine pulling out a syringe plunger with the nozzle closed.
- higher revs mean higher rate of fuel burning. Hence more fuel is consumed. However, if you go sufficiently fast at high rpms, you reach your destination faster thereby getting a good kmpl reading
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1. 50% throttle 2500rpm 80kph 4th gear OR
50% throttle 2000rpm 80kph 5th gear
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Since throttle opening is same, we can ignore that. And, since rpm is lower in 5th gear, the second option is better in FE.
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2. 50% throttle 2500rpm 80kph 4th gear OR
40% throttle 2000rpm 80kph 5th gear
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Since both throttle is opened lesser in second case, it depends on engine size. Big engines (1.6 and above) have fairly large intake manifolds. Hence, even at 40% throttle, the resistance to piston is not that high. However in smaller engines (800cc, 1.2L etc.,) 40% throttle could be some work for the pistons.
Another thing that we should consider is the gear box design. If the 5th gear has overdrive, then there will be better FE.
Anyway, I think, the second option still has better FE here.
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3. 40% throttle 2500rpm 80kph 4th gear OR
50% throttle 2000rpm 80kph 5th gear
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The first option is kind of better since, the throttle is more open in second case which means, the engine is more loaded. Maybe you are going up a gradient or carrying more weight and that's why the rpm has gone down. Maybe you will have to shift down to keep the vehicle moving at the same speed any moment.
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Now, we know that a turbo works by drawing in more air into the engine by using energy from exhaust gases.
But if more air is being drawn into the engine, won't the ECU be forced to compensate by pumping in more fuel, thus increasing fuel consumption?
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Well, in case of charging, you are partially correct that forcing more air leads to more fuel injection. However, there are some advantages. By pressurizing the intake manifold, throttle losses can be reduced. The exhaust back pressure can be reduced. More torque can be produced, especially at lower rpms as well. Also, the volumetric efficiency of the engine would be higher. Thus, we can't directly compare a NA with TC engine. Its based on how its used.
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Car 1 has a 3.0liter V6 running at 30% efficiency (rpm doesn't matter)
Car 2 has a 1.2liter I4 running at 30% efficiency (rpm doesn't matter)
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A 3L engine running at 30% efficiency by rule of thumb consumes more fuel than a 1.2L engine at same efficiency! However, if you consider a constant torque or power output, then, depending on the operating point, the FE will vary. Here, rpm also would come into play.
If you remember, in Top Gear once they had tested a Prius (hybrid) and a BMW M3. The Prius was driven at its limit and the M3 was following at same speed. Here, the Prius, although a smaller engine consumes more fuel since its run at almost max rpm. i.e., it is producing its max power. However, for an M3, the max power of the Prius is probably its cruising power and hence it consumes much lower fuel in spite of being a bigger engine. Hence, the operating point is vital. An engine has its most fuel efficient operating point and if driven there, it will give best FE. However, for best torque or power, you need to be at a different operating point which will invariably consume more fuel.
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And what exactly are the brake specific fuel consumption graphs? Can they be applied to real life?
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Plainly said,
BSFC is fuel consumed to produce a certain power.
For any engine, power produced depends on its rpm. Thus, you can plot a graph of power vs rpm and you can measure the fuel consumption (generally in grams per second) at each power or rpm interval and mark it on that graph. Thus, irrespective of the engine size and other dynamics, you can get a graph of power vs rpm and know how exactly one should use the engine to get best power or FE. It is quite practical for real life, but, no one gives these details.