0
$\begingroup$

It is common practice to drive at constant speed as much as possible to reduce fuel consumption, or at least to avoid "pumping" on the throttle (at frequencies high enough that the lowpass filter does not absorb it) but why?

Obviously accelerating then braking is just a waste of energy through heat, but what about simply stopping accelerating (since engine losses are there whether one accelerates or not)?

Application example

You're on the slowest lane just under the limit, when you get closer to a truck in front of you. You can't overtake so you have to stop acting on the throttle and lose 20mph with the engine brake alone waiting for an opening. When it's clear, you accelerate a lot to get back to cruise speed; then repeat the situation several times. Why is it inefficient?

Potential solutions:

1) Similar behaviour of the internal combustion engine to electric DC motors where the (Joule) losses are proportional to the torque. Why is that so?

2) ?

$\endgroup$
2
$\begingroup$

You can't overtake so you have to stop acting on the throttle and lose 20mph with the engine brake alone waiting for an opening. When it's clear, you accelerate a lot to get back to cruise speed; then repeat the situation several times. Why is it inefficient?

Presumably this occurred at highway speeds, where hurry up and slow down is very inefficient. This hurry up and slow down driving can be amazingly fuel efficient at slower speeds. One alternates between accelerating (at some optimal value) and then coasting. The fuel injection system shuts down fuel flow during the coast phase. Another name for this is pulse-and-glide.

The reason hurry up and slow down is inefficient at highway speeds is because your vehicle is already operating beyond it's peak efficiency. Internal combustion cars and light trucks experience their best mileage somewhere between 35 and 55 mph (55 and 90 kph), depending on the vehicle. The lower end is where trucks and SUVs operate; the upper end is high end sports cars. Unless you're stuck in a traffic jam, your freeway speed is faster than your optimal speed.

Accelerating to pass does a number of things to reduce fuel efficiency at these speeds. Drag grows quadratically with velocity, so even a small change in velocity increases drag considerably at highway speeds. Your car doesn't have much oomph at highway speeds if you don't downshift, which makes acceleration rather expensive fuel-wise. If you do downshift, that throws your car into a regime where torque falls with increased engine speed. Your car now accelerates nicely, but at the expense of seeing the fuel needle move.

So how does hurry up and stop ("pulse and coast") work at slow speeds? Internal combustion engines are rather inefficient with regard to producing torque at low engine speeds. Fuel efficiency suffers as a result. A typical car is considerably more fuel efficient when operated at 45 mph as opposed to 30. Suppose you accelerate optimally from 30 mph to 40, then let off the gas completely and coast back down to 30, from which point the cycle starts anew. The increased speed will cost a bit in terms of increased drag, but drag isn't nearly as strong a force at 40 as it is at 60. You'll more than make up for that drag loss with improved engine performance experienced while accelerating to 40, and your car consumes no gas during the coast phase.

There is one minor problem with this pulse and coast technique. Other drivers do not appreciate it at all.

$\endgroup$
3
  • $\begingroup$ Good answer. Between two scenarios, (1) Throttling at the optimal speed (say 70kph) without acceleration/deceleration, and (2) pulsing and coasting between 60-70kph, assuming level road, which is more fuel efficient? @David Hammen $\endgroup$ – t.c Oct 1 '14 at 18:24
  • $\begingroup$ Apparently the latter. BTW, I had the name wrong. It's pulse and glide, or burn and coast. I'd heard of this technique before, but I don't do it. This sounds like a very good way to get other drivers on the road very upset. And police. This is apparently illegal in some areas. $\endgroup$ – David Hammen Oct 1 '14 at 19:19
  • $\begingroup$ Interesting answer, thanks. So there isn't anything about the engine efficiency decreasing with torque for constant speed (whether we stomp on the pedal or not)? $\endgroup$ – Mister Mystère Oct 1 '14 at 22:03
2
$\begingroup$

There are a lot of things in play here. Certainly ICE(internal combustion engines) are less efficient when accelerating than when running at an optimized RPM.

However, the interplay between external drag, gear ratio, and engine speed is rather complicated. Typically there's an optimum speed for maximum efficiency; higher speeds have too much air drag and lower speeds run in a slower gear and thus require more engine RPMs.

Even so, running at constant speed is not necessarily the best approach. You may be better off allowing the car to slow down on an uphill, and (warning: not safe and possibly illegal) disengaging the transmission to "freewheel" downhill, for example.

$\endgroup$
6
  • 1
    $\begingroup$ Except that when going downhill it actually saves more fuel when the engine is in gear (since engine braking cuts off fuel supply, while letting the engine run on neutral consumes fuel). $\endgroup$ – t.c Sep 30 '14 at 12:53
  • $\begingroup$ @t.c Interesting: do you have a citation? Typically engine braking does just that: blows off a ton of kinetic energy, so I'd be surprised if the fuel savings exceeded the energy loss. $\endgroup$ – Carl Witthoft Sep 30 '14 at 12:55
  • $\begingroup$ Yes engine braking wastes kinetic energy so you get less speed but since it uses less fuel than coasting the fuel economy is better. quora.com/… forums.anandtech.com/archive/index.php/t-1930756.html $\endgroup$ – t.c Sep 30 '14 at 13:03
  • 1
    $\begingroup$ Thanks for your answer. I second @t.c's answer, the calculator cuts off the injection when the user does not accelerate, the power to keep the engine running is acting like a brake. 0.6L/h freewheeling, 0L/100km in gear downhill says my Zafira's computer. Aside from the fact the engine is more efficient over a range of rates (which I'm interested to know why) and so is the entire system regarding speed, it sounds like considering a small increase in speed accelerating a lot is very power hungry and I'd like to know why. I've updated my post. $\endgroup$ – Mister Mystère Oct 1 '14 at 9:36
  • $\begingroup$ If you have fuel injection -- I suppose nearly all cars built in the last 30 yrs or so do :-) $\endgroup$ – Carl Witthoft Oct 1 '14 at 11:43

Not the answer you're looking for? Browse other questions tagged or ask your own question.