Fuel economy

[mynameisowen]

This is interesting because I have always "wanted" to run at a higher throttle opening to eliminate pumping losses however doing so means you rev substantially higher.. Doing what you said about 50mph in 4th gear sounds absurd to me since I'm normally in 5th by 35mph. I can see how it would work but personally I don't think revving that high would be worth it, you must be at like 3k rpm no? Thats higher than I ever go on my daily driving. Its pretty hard to compare car to car because of all the different factors that come into it, my results are pretty good and I would change my driving style to how you are describing to see if it makes a difference but I can't risk wasting my money on fuel!

On a related note: I wish some manufacturer would design a PETROL car where the car is constantly at WOT or close to it and the engine speed is controlled by the amount of fuel injected (like a diesel). This would eliminate pumping losses which are HUGE at low speeds and your engine would effectively be on lean burn with crazy air to fuel ratios. Combined with direct injection and some fancy maps I reckon this is the way forward. Anyone have any thoughts??

[Dodo Bizar]

Really, 5th gear at 35 mph does not give you good mileage. If I would drive 50 mph in 5th gear I hardly crawl over 2000 rpm. Doing 50 mph in 4th gear is still below 3000 rpm.

You mention pumping losses, indeed that is one part of the equation. The other part is simple the efficiency of the combustion which is very low at low revs due to relatively too much overlap in the cams (perhaps my Skunk2 cams exaggerate this). Depending on the exact setup the most efficient point is somewhat a compromise between all the losses. Also depending on power output. Generally with more power requested, the most efficient point to rev goes up, with the maximum efficiency being very close to the point of max torque when WOT power output is considered.

Ah well, in the end its the amount of fuel injected which I have monitored in the past and the ZT2 display which tells me a fair lot about what is going on. Just try it for a while to rev a bit higher, but make sure the ECU stays in closed loop. And pumping losses... well I think in the end I just rev a little higher, shifting between 3000-3500 when I am really lazy driving. I would not consider that a very high revs actually .

[saxophonias]

we have to bear in mind a typical dynoplot of a mid tuned d14. The torque is high up to 3000rpms from which point it starts falling a bit and then keeps rising again. In combination with the d14a8 gearbox which is shorter i have noticed that if i stay up to 3000rpms it is really torquey in the road almost in every gear. So i can accelerate sufficiently or maintain speed easily with only 7-12%of throttle.

[mynameisowen]

Good discussion My thoughts:

The engine operates most efficiently at peak torque. But we dont all drive around at peak torque because then we would be driving at ~4000 rpm and to do this we would need to be in a low gear to match the speed we want to be at. This is what you are describing no Joris?? At these high rpm's the losses in the engine would be high and even though the engine itself is operating at peak efficiency, you could maintain the same amount of road speed whilst using less fuel (which is what we are talking about here).

Pumping losses in the engine come from the extra work the engine has to do to "suck" the air in. The engine has to suck because when the throttle is partly clossed the pressure in the inlet manifold goes below atmospheric. These pumping losses decrease with a larger throttle opening. So I assume this is where the main benefit of driving at higher rpm's comes from, and is the reason why diesel engine's are so much more efficient than petrol's, because they have no throttle.

Frictional losses on the other hand increase with engine speed since as the engine rotates faster more work will be required to overcome the friction of all the bearings, etc.

I 100% agree with your reasoning, I just don't think that in normal road circumstances *on a stock car* driving at higher rpm's will give you a benefit, I think the benefits would be outweighed by the increased losses in the engine. Am I right in saying that your car has a more aggresive cam aswell?? If so thats probably the main reason you get better economy driving at higher throttle opening than me since the pumping losses would be so severe.

I agree that 35mph at 5th gear isn't a very good place to be but to be honest I'm only ever at that speed for a short while. E.g. when I'm cruising down a slight incline where I know I'm going to have to stop soon. On an open road I'd rather be at ~45mph (5th gear), at this region I reckon you probably get the best fuel economy. All I know is with the way I drive, on a good week I get 50mpg, there are probably a few different combinations of driving methods to attain good fuel economy given that every method is a compromise off something else. What we need to find out in this thread is which is the best for our car to promote money saving and engine life!!

I've never really considered the engine being open loop at low rpm's though, when is the cut-off for this (if there is one)?

[BETEK]

The most fuel consumption is going in the acceleration process.
When we are cruising we need to keep the speed stable with the less engine revs for one proper speed without stalling the engine.
This achieves with the longest gear we have for that speed for every road condition. For example if we are cruising at plane road with 100kmh we will have 5th gear, if we cruising with 40kmh we will have 3rd gear.
But if we are climbing a hill maybe we will need for 100kmh 4th gear and for 40kmh 2nd gear.
So all the tricky part to fuel economy is to adapt the car speed to the road conditions and accelerate smoothly and slowly without stalling the engine.
If we stall the engine the fuel consumption increases.
When the throttle are going to open widely at cruising it seems that the we want to keep the cruising speed stable. And from that point we are entering the acceleration process. Now it depends from us to select the correct gear to keep the cruising speed and have the throttle us much we can closed (without stalling the engine) for that proper speed we want for cruising.
And when we retrieve the proper speed we want for cruising we select the longest gear for that speed and have the throttle us much we can closed to prevent the big fuel consumption

[Dodo Bizar]

I think we are all on the same line here.

Indeed my EJ9 has this aggresive cam. But my girl has a bone stock EJ9. I always get better mileage than my girl with her car, because she gets sloppy and shifts too soon while pressing the throttle too much after a while.

The open loop/closed loop switch point: I believe the main switch is MAP and TPS. At certain thresholds (which kick in really soon) the ECU decides to go open loop. When I drive up hill on a small inclination my ECU immediately goes to open loop when in 5th gear doing 80 kmh. Indeed here is one of the tricks why high revs work, it helps you maintain closed loop mode while accelerating faster compared to higher gears.

And about the efficiency point... I mean it in a complicated manner. For each requested power level, there is an optimum point. At max throttle this optimum is indeed near max torque, but at lower power levels you need to go down in revs for optimum. This is what Betek actually says about the different gears at the same speed due to different load request due to different slope.

Optimum revs at no power request are obviously around idle rpm, so between no power take off and full power take off the optimum shifts from idle rev to max torque rev is my hypothesis. Exact optimum for each car and circumstance will be different.

Btw Honda did provide our cars with a shifting table, everybody check your booklets that came with the car if you have those. But it seems silly to me. For all different D-series you get the same table... it says shift at 72 kmh to 5th gear from the top of my head . Well obviously I disagree with that statement, but wanted to mention it.

[saxophonias]

i am going to check my apexi from which point and on (%)the ecu changes from open loop to close loop. I think it is around 40% or something.

[mynameisowen]

Interesting about the open/closed loop thing. I mean I would happily shift down a gear so that my engine is running closed loop but we don't know when that cut off is. If it is purely on the manifold absolute pressure and throttle position sensors then that will surely be unrelated to engine rpm and rely on throttle position. If so, it doesnt matter what gear you are in, but to ensure that you dont speed up too much when you dont want to, you would have to stay in a low gear (like how you drive Joris). If this is true then it would seem it favours your argument, but I still think, and most information and fuel economy states that you should keep engine revs low, wether this is because it would be too complicated to explain to the general public how lambda sensors and ECU's work remains to be seen

I always thought the shifting table was too high and you think its too low

Betek I agree with you about being in the longest gear possible (thereby maintaining low engine rpm's and saving fuel (?)). But what Joris is saying about being in open loop at low rpm's may counteract what we are both saying. It would be good to know the precise conditions where the changeover lies.

That would be good Polyvios, by percentage do you mean %throttle opening? Because that seems quite high for daily driving, but I could be wrong (since I accelerate quite slowly)...

[saxophonias]

guys my conclusion for economy is to push the gas pedal as little as possible in order to accelerate or maintain the desired speed. What i was saying in my prior post is after which point the 02 sensor stops messing with a/f ratio.

[mynameisowen]

Joris can you clarify the conditions that are required for running in closed loop?