Opinions/advice on speed and acceleration

[wungun]

At certain highway speeds, i notice my car seems more efficient...
I know speed kills efficiency, however if im going too slow, say 70km/h, my instant fuel mileage can be painfully high. Especially when running in to an incline...
We've all noticed when speeding down a hill with a steady throttle how much the extra speed helps instant economy figures.

What advise should i adhere to for example, approaching a hill?
I typically ease up (target) on the throttle on hills to keep my mpg from going to high, but as i jusg mentioned, slowing too much kills me.

And advise on acceleration...for example, after rounding a corner and wanting to get back up to my "happy" speed...
Easy slow acceleration? Quick thru the gears, 70% throttle to target speed?

Thanks

[JRMichler]

You will need to find for yourself what works best with your vehicle. Ultragauge or Scangauge is your friend.

My truck gets about the same mileage whether I accelerate about the same as most other cars on the road, or accelerate like a granny. So I just step on it and go. I do, however, pay a lot of attention to DWL and DWB.

[UltArc]

I prefer to drive at 60% load, keeps me always above 30 mpg, but when I am traveling under 35 mph, and need to jump to say 50 mph or higher, I down shift and get there. I think if it will take too long under a comfortable mpg, then it is usually easier to just get there, rather then move slowly.

But then one has the traffic variable. If there is dense traffic, I move slower- unless its so dense I am getting better air flow. If there is no traffic or in the city, I move slowly, if there is some or I am blocking the way, or if there is little to no chance of needing to stop soon, I just hurry up and get to speed.

As JRM said, you will find your own settings. It takes a while, but experiment, try new things, and you will start to get an idea on what works best for you, your vehicle, and the conditions.

[Fat Charlie]

It's good to see you've got a manual; that complicates things but gives you more potential. First, ignore speeds (as targets) and focus on rpm and gears- 70kph is a fake number because you didn't say what gear you were in. If your most efficient engine speed for high load is, say, 2000 rpm, make that your target and be in the right gear to climb the hill at something close to that. Every hill is different, weather will play a factor and traffic conditions have a lot to say about what you can effectively do, but focus on the tach more than the speedometer.

Originally Posted by wungun

We've all noticed when speeding down a hill with a steady throttle how much the extra speed helps instant economy figures.

You know what helps even more? Doing that in neutral. P&G FTW.

[Occasionally6]

Yep; rpm and load. If you don't have a manifold pressure (vacuum) gauge, throttle position (a Voltmeter on the throttle position sensor output will do) is a reasonable proxy.

It's all about managing the energy flows. Extract it from the fuel as efficiently as possible and store, and extract it from the store, as efficiently as possible. You can store the energy as kinetic energy - road speed - or as potential energy - such as when climbing a hill - or, less efficiently, as a cold sink in the A/C (if you use it) and chemical energy in the battery. Everything else is just loss or used immediately.

If you look at any of the BSFC maps they're all much the same shape with moderate-high load and low-medium rpm describing the most efficient operating range. Climb the hill or accelerate with about that 60-80% throttle, 2-3000rpm and get the high engine efficiency. You will then be both on the throttle for less time, operating efficiently when you are, and off the throttle for longer (vs a slow acceleration or climb at light load), using the stored kinetic or potential energy to keep the car moving.

Yes, coasting with the engine idling will consume less fuel sometimes. If your road speed drops too low you will actually be better off in gear and driving. (Idle is an extremely inefficient engine load and speed, it's just that motoring, i.e. coasting in gear, is worse.)

If you are braking anyway (you should be as little as possible of course) keep the car in gear, off the throttle, and allow the engine to be motored by the car. That's kinetic energy recovery.

It's even annoying sometimes to be following someone driving conventionally and therefore being unable to accelerate at a more efficient rate. In this case I find coasting to make a space in which to accelerate works.

[Xist]

Originally Posted by Occasionally6

Yes, coasting with the engine idling will consume less fuel sometimes. If your road speed drops too low you will actually be better off in gear and driving. (Idle is an extremely inefficient engine load and speed, it's just that motoring, i.e. coasting in gear, is worse.)

I think that I know what you mean. When I have pulled over on road trips I will coast as far as possible, but there does come a point when my instantaneous MPG drops below what I could get while driving normally, so I stop coasting.

Wungun, I will just mention that, in theory, your engine should not use any fuel when you coast downhill, thanks to the magical, mystical, and even mythical DFCO, but that does not seem to be reliable. Your Fiesta has a built-in MPG gauge? That is good, but they tend to be slightly inaccurate, and do not provide as much information as a Scangauge or Ultragauge.

The best thing to do might be testing--drive the same hill several times in different ways and log your readout.