Driving Economically.

Joe_Horner

Depends on the speed limit

But you'll do best by getting to whatever speed you choose - whether that's 70 or 56 or 88.8mph - as briskly as possible while not hitting anyone and using the gears to keep as close to peak torque as you reasonably can!

mikey72

Joe_Horner wrote: »

Depends on the speed limit

But you'll do best by getting to whatever speed you choose - whether that's 70 or 56 or 88.8mph - as briskly as possible while not hitting anyone and using the gears to keep as close to peak torque as you reasonably can!

Again though, easy one first, I've one car with a twin barrel carb.
Accelerate gently, only using the first barrel, economy is mid thirties.
Open the second barrel, and the economy goes down to less than 20.
Accelerate hard from a low throttle opening, and the
accelerator pump delivers fuel like it has shares in Shell.

So accelerate gently, over a longer period, a lot less fuel than accelerate hard over a shorter period.

My modern fuel injected car appears to have the same characteristics. Violent acceleration, more fuel used.

Strider590

Joe_Horner wrote: »

With respect, the bolded bit is a nonsense statement. You also can't obtain "full power and torque" at the same time

"Peak torque" may or may not be at full throttle depending on the conditions. You simply can't compare the two because the peak torque speed is an engine speed, and the speed an engine happens to run at depends on load and gearing to the drive wheels as well as throttle opening.

However, the maximum efficiency of any engine operating (approximately) on the Otto cycle will always be at whatever engine speed produces maximum torque for that particular engine and with a wide-open throttle.

That's nothing to do with the ECU, it's a simple fact of the basic principles involved:

The thermal efficiency for an engine is highest at the peak of it's torque curve and the volumetric efficiency is highest at full throttle. So, when the two conditions coincide - peak torque RPM and full-open throttle, the engine is at it's most efficient overall.

You can't maintain that condition while accelerating with a conventional transmission because the engine speed will increase as you go faster, until you change to the next gear. But you can get very close by using short changes and keeping the revs around the torque peak while having the throttle fully open to maximise volumetric efficiency.

If you doubt that, consider what a clogged air filter does to fuel consumption, and that's a far smaller restriction to airflow than a closed throttle

Firstly I didn't state that peak torque and power occur at the same point, I merely hinted that to achieve either you need to apply the throttle progressively.

Secondly, full throttle is throttle valve fully open with maximum fuel/air flow and throttle position = engine speed.
If peak torque is at 5000rpm, you don't need to open the throttle to the position of 9000rpm and doing so by simply standing on the go pedal, would not give you good fuel economy at all. The key is to have a feel for the car and control your right foot to push fuel/air into the engine at the rate it's needed.....

Side note on that, you can always have fun and give it too much, then back off sharply to get the thing spitting flames and popping on the over-run as the excess fuel passes through into the exhaust manifold and ignites....

This ^^ is why is mention ECU's, because modern cars with "fly by wire" throttles, tend to behave the same whatever you do, because the ECU controls the throttle valve to open up at a preprogrammed maximum rate if you simply stomp on the gas.

I think this is why people don't understand this principle, they've been mollycoddled by modern technology and forgotten how to drive.

Joe_Horner

Accelerator pumps on carbs are an odd beast because they're basically a bodge to get around problems with fuel dropping out of the airflow in the manifold. That can drop the air fuel ratio at the cylinders to 20:1 or worse for a second or two, which will barely burn. The fuel that drops out of the air will re-evaporate and be burnt within a few seconds but it still causes a sudden weakening on fast throttle opening, which the pump adds extra to compensate for. So whatever fuel the pump delivers is effectively "wasted".

Bear in mind that most accelerator pumps still deliver the same amount of fuel as you open the throttle regardless of how fast you put your foot down, even though it's generally not needed with a gradual opening. That's one of the reasons that CD carbs like SU and Stromberg tend to be slightly more efficient than fixed choke ones - the enrichment is done by damping the piston and the damping effect varies depending on how fast it tries to open so you get very little extra fuel with a gradual throttle opening. Depending on the type of injection (single or multi-point) fuel injection needs very little, if any, enrichment on acceleration - it's a matter of 3 or 4 AFRs to move from "max economy" (typically around 16.5:1) to "max power" (about 12.5:1) mixture strength

Anyway, swerving back on track, the problem you have with a big twin-barrel is that it'll dump that extra fuel every time you open the throttle so, if you go for short, sharp, gear changes to stay in a good rev range, you'll be pumping 3 or 4 strokes of the accelerator pump into the carb instead of 1 or 2 by accelerating more gently in a couple of gears unless you throw in full-throttle gear changes (which the car won't thank you for :P ). That may end up using more fuel from the accelerator pump than you save by keeping in an efficient range.

On fuel injection you may see the instantaneous consumption drop alarmingly as you floor it but, if your brain is quick enough to integrate the time / consumption figures and compare with a gradual acceleration, you'll find that the quick version has actually used less fuel if you don't go far past the peak torque revs in each gear.

Bear in mind that the torque curve of most engines is pretty asymmetric - it gets close to maximum at fairly low revs but drops off quite sharply after it peaks. Hence pulling to the red-line in each gear is BAD cos you're using wide throttle at an inefficient engine speed soon after you pass max torque.

Delivery_Driver

Just to add another spanner in the works of point 7, I had my car mapped last year on a RR.Car is put through its paces and when they get to peak power then its allowed to run down as not much point holding high revs for no gain.Peak power was at 6k rpm, peak torque at 5500rpm(quite a large turbo).
The way the car on the rollers is run is quite similar to how point 7 describes to accelerate, plus there is no aerodynamic drag.The result was 17 miles travelled on 1/4 tank of petrol, so about 8mpg.

Joe_Horner

There's no aerodynamic drag on a rolling road but there's a HUGE amount of "artificial" mechanical drag because that's how the RR works. It's like driving with your brakes hard on

Delivery_Driver

Good point, I was overlooking the load they put on the wheels to make the engine work hard.
Well ,lets assume the aerodynamic drag and mechanical drag put on the car by the rollers is the same.It still only did 8mpg.

Notmyrealname

It isn't. Aerodynamic drag increases by a square of the value as the speed increases. RR mechanical drag is a constant.

On the course I did, we were shown a graph of wind resistance vs speed for that lorry. Basically very little until about 53MPH then it increased exponentially.

Joe_Horner

Strider590 wrote: »

Secondly, full throttle is throttle valve fully open with maximum fuel/air flow and throttle position = engine speed.

But throttle position doesn't equal engine speed. Engine speed depends on throttle position and load. If you're driving on the flat with a steady throttle position then start to climb a hill your speed (and therefore your engine's speed) will drop if you keep the same throttle position. So full throttle isn't "the position of 9000 RPM", it's the position of "maximum possible RPM for the load".

Even without that, what matters for engine efficiency is the speed the engine is actually doing, not "what it can do / will end up doing" at that throttle opening. Unless you have a badly slipping clutch (or a CVT) your engine will not shoot instantly to peak RPM as soon as you open the throttle wide.

Joe_Horner

Delivery_Driver wrote: »

Good point, I was overlooking the load they put on the wheels to make the engine work hard.
Well ,lets assume the aerodynamic drag and mechanical drag put on the car by the rollers is the same.It still only did 8mpg.

The drag from the rollers is "whatever's needed" to use up every drop of power the engine can produce at a given speed. It's as if you were driving at full throttle and wanted to sit at (say) 50MPH, so you kept the throttle wide open and shoved your foot on the brake hard enough to keep your speed down to 50.

Imagine what that would do for your fuel economy!