Footman

iTrader: (2)

This is a technical question that I hope someone more knowledgable can explain to me.

Let's say I'm driving at 60km/h in 6th gear. The RPM's are something like 2000 - 2500 rpm.

If I mash the gas pedal, you all know at that speed and in that gear, the car won't accelerate much faster. My question is, despite the me mashing the gas pedal in at WOT, but my engine is sitll at around 2500 --> 3000 rpm, am I still consuming the full amount of gas at 100% throttle?

What I'm trying to get at is that my throttle is completely open, but the engine is still at around 2500 - 3000 rpm, so is gas consumption the same as say 4000-5000 rpm travelling at a faster rate but holding steady with a very light throttle?

To my understanding, the throttle is what controls the amount of fuel that goes into the engine.

I'm not sure if I'm very clear on what I'm asking at, please tell me if you don't understand what I'm saying, I'll try again.

Cheers,
Andrew

Spin9k

I'd guess that you are not correct. It's a fly-by-wire throttle, which means it does what the ECU tells it to do, i.e, what is correct and sufficient for the engine parameters at hand regardless of what you do with your foot. Your foot by the way is connected to a potentiometer only, with wires to the ECU.

Your example of trying to accelerate at low rpms might not provide the best economy for the car, but that's another matter.

Footman

iTrader: (2)

That doesn't answer the question on gas consumption.

Since the RX-8 has drive by wire, then so given that situation, what will the ECU do to the throttle (intake manifold) going at 60km/h, in 6th gear and flooring the car.

Spin9k

It is difficult to answer, it will open the throttle some, but not completely likely, but enough to have the injectors supply more fuel to attempt to fullfill your request to go faster, but no more than the engine can utilize without running overly rich.

Remember this is not a carb, air is administered to the engine by the throttle body, gas by the injectors, both controlled by the ECU to optimize operation to tables within the ECU.

expo1

It should flash a light saying " DOWN SHIFT INTO 3RD!!! "

The ECU knows what gear your in and how fast your driving. In your scenario only the primary injectors are working so it doesn't matter how hard you floor it. There is only so much fuel that will enter when running on just the primary injectors. The reason a downshift works the way is does is that your RPMS get into area where the 2nd and 3rd set of injectors open and thus give the engine more fuel.

Jax_RX8

This is mostly correct.

To relate this to one of the the questions - the engine would use less fuel at 2500 RPM at Wide Open Throttle (WOT) than it would a 8500 RPM WOT because the engine can only intake so much air at that RPM and the ECU will only allow so much fuel to be provided for a maximum air to fuel ratio (richness) for the air available.

But, the throttle body (allowing air into the engine) will open fully upon on WOT to allow maximum air flow into the engine, but the ECU will control fuel to match the air available through the intake up through the RPMs.


The other question - Cruising at low RPMs (high gear) versus higher RPMs (low gear) and traveling the same speed - which uses less fuel.

Answer is - it depends.

Normally they middle of the RPM range, where the engine develops 90% of peak torque (around 2500-5500 for the 8 IMO) is the most fuel efficient range - and anywhere in here is about the same.

But if you lug and engine it will use more fuel (very low RPM with little torque, below 2000) or rev it much higher than necessary for the speed traveled (over say 5500), it will use more fuel than in the middle of the RPM range.