If you do follow all those “hypermiling” tips, you still only save maybe £200 a year, even here where petrol is extremely expensive. I tried it for a while a few months ago (not following any particular programme, just doing common sense things which it turns out are pretty much in line with what the hypermiling link suggests), and I got my mileage up from about 36mpg to 41mpg (British gallons, so mpg figures are 20% higher than their US equivalents). Do the arithmetic and it turns out that all that self discipline earns you just £4 a week.
Not less, on deceleration a modern fuel injection system uses no fuel. The injectors shut off 100% when you lift your foot off the gas. There is no advantage to injecting fuel on deceleration and several disadvantages to doing so, primary decreased fuel mileage, and increased pollution.
A cite, I got your cite right here. from the Robert Bosch Gasoline Engine Manual published by the SAE
So putting your car into neutral will cost you additional fuel. A small amount to be sure, but additional.
Figures to prove this? I doubt it, but I can tell you from both a technical and practical aspect that floorboarding the gas will cost you additional fuel. Again from Bobby Bosch’s book
From my own teaching material that additional fuel can be up to a 1/3 longer injector pulse. So instead of say a 9 ms injector pulse, it could be 12ms.
Accelerate slowly, and reap the rewards. If yo doubt this try jackrabbit starts for one tank of gas, and you will become a believer.
Maybe. Again based on my experience, I would have to ask, for how long are we leaving it off? This question came up before, and after some serious thought, I concluded that starting a warm engine is about the same as idling for about 30 maybe 45 seconds. However fuel usage at idle is pretty low.* IMHO unless you know you are going to be there for more than 2 minutes, I don’t think it is worth it personally. Shutting off your engine will cause additional wear on the starter, battery and other devices in your car.
*My Volvo (and most of the other ones also) burns about 1 kilo of fuel per hour. Using my scan tool, I can monitor the mass of air flowing into the engine. At idle that is about 13-14 Kilograms of air per hour. Put the car in gear, and we will say round numbers 14.7 Kg of air per hour. At idle the engine will run at 14.7:1 so it will burn roughly 1 kilo of fuel. Again roughly let’s say 2.5 quarts of fuel. That is 80 ounces. Or about 1.3 ounces per minute.
So why did you stop? What benefits do you gain from spending £200/year more on gas?
Transmission braking in a manual has it’s uses, but I usually neutral it to a stop, too, my logic being that it’s cheaper to replace brake pads than a clutch.
Of course, YMMV.
I have continued with some of the mileage-enhancing practices, such as leaving a large gap in front in slow moving traffic to eliminate stop-start. I reckon that that is worth doing because it is less stressful to both car and driver. But otherwise, driving for maximum mpg involves a lot of tedious planning ahead and restraining of natural instincts. It’s quite an effort.
I don’t find it to be a chore. I follow all of those tips with the exception of turning off my engine at sop lights.
I always get more than the EPA estimate mileages on my cars.
I do not drive slow, I just don’t floorboad the gas to get there.
It’s practice and a habit.
All I know is that after three months of nursing my mileage up from mid 30s to above 40mpg, it was a blessed relief to kick back and have a bit of fun while driving. I’m back down to upper 30s, which is better than before and I’m happy with that. I’m still a more fuel efficient driver than a lot of people out there, who rocket past me only to brake at the traffic light that was already red.
Thanks, Rick, that seems pretty definitive. The bits I understand, anyway…
I shall no longer coast in neutral, certainly. And accelerating slower has already reaped its rewards, small though they may be. And there’s not much point in turning the engine off if I’m going to be stopped for less than a couple of minutes.
But if you’re going to be coasting a mile or more, couldn’t eliminating the engine braking factor by being in neutral more than make up for that?
Um, I hate to even quibble about this since you pretty much are the board mechanic god but how does the engine keep running if all fuel is cut off? I could understand maybe only triggering injectors less often at idle since inertia will keep things going nicely even if only firing every few revolutions on a reduced number of cylinders but zero fuel?
I would disagree with this position in some, fairly uncommon situations but I will defend that in terms of gas mileage, going into neutral while coasting downhill is appropriate if gas mileage is the primary concern.
As a hypothetical using some out there numbers, let’s say that it costs my engine 0.5 kW to idle at 700 RPM. I’m driving downhill on a slight incline of 2% that happens to be perfectly matched to coasting in neutral to keep me the speed I want to go, needing neither braking nor occasionally dropping into gear to get back up to speed. If I were to put the car into 5th gear, the engine would be running at let’s say 2200 RPM. For the purpose of this scenario, let’s say there is no net accelerating or deaccelerating force from the transmission to the wheels. Now the engine must contribute 2.0 kW of power to keep itself from slowing down the car. So, there’s a net power-savings of 1.5 kW in this situation, a small amount of gas. Unlikely to come up in most driving, but if I have a very long downhill with no traffic where I feel comfortable driving at the speed my car will reach through cruising, I think it’s defensible in terms of saving gas.
On a hill, how does gravity strike you as an answer?
Gravity is pulling the car down the hill and driving the wheels. The wheels turn the driveshaft (On a RWD)/ axleshafts (FWD) the transmission gets turned by the driveshaft or axle shafts The transmission turns the engine.
On flat ground it is inertia. Good old Mr. Newton and his object in motion wants to stay in motion.
If you are in neutral it is again inertia, only this time it is the inertia of the rotating components inside the engine.
It might help if we look at the parameters on a typical engine.
As long as the engine has gone above 1800 RPM when you lift your foot off the gas, fuel will be cut off until the engine reaches 1200-1500 RPM (depends on temp of the engine)
so the fuel comes back on before the engine can stall. It may only stay off for a few seconds at a time, but it adds up.
control-z that would depend on the hill. About the only place I ever do this is on Interstate 5 North where it drops into California’s Central Valley. If traffic is light, I have been known to kick my car out of gear about 2/3 of the way down the last 6% down grade, and just let my speed build. At the bottom of the 6% grade it still goes down at at about 2% for several miles. With luck, I can coast for maybe 3 miles before my speed drops. While I have no cite for it, my automatic transmission acts like it does not care for this practice. This is one reason I only do it in that one location.
The problem in traffic is three things happen every time you touch the brake pedal:
- You slow down
- You are wearing the brake pads out
- You are wasting the gas it took to get you to speed.
So the trick is to never hit the brake. Or if you have to use it, use it as little, and as gently as possible. So if you kick your car into neutral, and then have to hit the brake you have a double whammy, you are burning gas in the engine, and via the brake pedal.
Are you sure you quoted the right part of my post? The part you quoted is discussing fuel injection design. From an MPG / emissions point of view there is zero advantage to putting fuel into an engine that is decelerating. Of this there can be no doubt.
You say you disagree with me but your entire post argues about driving down the perfect hill, which is a different subject than fuel injection system design. Concerning the perfect hill see my answer to control-z
No it doesn’t. A car’s driveline works both ways. When you are going downhill in gear the wheels are spinning the engine. The engine is acting like an air pump with no fuel involved.
I apologize, I think I misinterpreted your post saying that there is no fuel advantage to put your car into neutral and coast in some situations, but obviously we agree according to what you’ve written to control-z. So never mind, we’re in perfect agreement about the, “perfect hill.”
Throatwarbler, I understand that when you are going downhill the, “wheels are spinning the engine.” However, in the “perfect hill,” that Rick and I are describing, the speed at which the car will coast downhill in neutral is a speed the driver is willing to drive at. If you make the inertia and potential energy of the car descending the hill do the work of keeping the engine moving, you’ll either have to add throttle or come out of the hill at a lower speed. The overall difference in work done by your engine (fuel usage) will be in the time and difference drag from the engine at 2,200 RPM or 700 RPM.
Your premises appears to be predicated on the engine using less fuel to maintain idle while you are in neutral than it would use to make up for the lost potential energy of the car keeping the engine idling. This seems to me unlikely.
How’s the injector fuel cutoff work on a manual transmission, then? Are there measuring devices on the clutch plate so that ECM knows when to start firing the injectors again? That is, if I’m in gear coasting, then supposedly the computer stops firing the injectors. But the instant I hit the clutch, it has to be fast enough to start firing again, right? I know US cars have clutch interlock switches so we’re not stupid and try to start the car in gear; but I know from (mumble-mumble) experience that the car I rented in Germany recently doesn’t have an interlock, so, no microswitch that immediately tells the ECM that I’m hitting the clutch, right?
One of the inputs to the ECU is engine speed. as soon as the engine speed drops to the cut in point, the injectors resume on the very next engine revolution.
No sensor on the clutch pedal needed. Hell you can push the lever into neutral w/o touching the clutch, it makes no difference.
Don’t forget we are talking digital electronics here. What seems instantaneous or magical to us is no big deal to a ECU. It probably takes the control unit 1 millisecond or less to turn the fuel back on. At 1200 RPM one revolution is 33 milliseconds. all the time in the world.
Okay, I’ll grant that I am making the assumption that the net drag from the engine (to be overcome either by fuel fed to the engine or input from the transmission) is considerably higher at 2,200 RPM than idle.
I simple cannot imagine why it wouldn’t be.
To begin with, chosing a lower RPM by shifting higher faster is a very well established and widely recommended technique to achieve better gas mileage. Another fuel saving measure is to select a lower idle speed because idling at a lower RPM requires less fuel consumption.
Breaking it down further, I wouldn’t predict that any of the drag forces working on an engine would decrease with speed. For instance I’m guessing that the force of scraping a piston through its cylinder is relatively constant meaning that the power it takes to do this increases linearly with RPM’s while the power required to displace liquids like engine oil increases exponentially.