That doesn’t work - on hills that are only moderately steep, engaging the engine, even in the highest gear, with no throttle will slow the car down - if I depress the clutch and coast down freely, the car maintains enough speed that it requires less energy to get up the next hill than it would if I’d allowed the idling engine to slow it down.
lowbrass describes it perfectly above in post #31.
It’s only part of my economical driving strategy - the much of it is simply to use the accelerator as little as possible (i.e. not accelerating into a situation where braking is likely), but as a whole, it makes a very significant and measurable (and measured) difference to the fuel consumption.
I don’t get this. 1. Surely an engine that is idling at 750 rpm is going to be using less fuel than one that is whirring round and sucking in fuel vapor at 3,000 rpm, even though the gas isn’t depressed; 2. Why should an idling engine with the clutch disengaged cause wear on the drive train? Does it go slack and whizz round with the wheels or something?
ETA: Mangetout, depressing the clutch causes lots of wear on it, and as my old man says, “brake pads are cheaper than clutches”.
If it involves cars, you can take any advice from Gary T. or **Rick ** to the bank. They’re our friendly neighborhood Jedi Master-level automotive service engineers.
Yes, but many modern fuel injection strategies will decrease, or even stop, injector pulse during deceleration mode, which means little or no fuel is being sucked in.
When you say “many”, does that mean not all modern cars will shut off the fuel flow when rolling down hill in gear without the accelerator pedal pressed? If so, which manufacturers do and which don’t?
Well shit. Nearly my entire knowledge of cars comes from British carburettor-driven vehicles (and that mainly because they went wrong all the time). I’ve been driving a fuel-injection Ford for seven years now and never realised that. Better re-think my driving style…
My what an interesting discussion this is. I admit to knowing not a single thing about the nature of car engines or their brakes. I don’t even drive.
I do, however, travel. And one of the places I have traveled is to India, specifically Darjeeling, beautifully positioned in the foothills of the Himalaya.
We took the toy train to the top, incredibly steep and winding and switching back, the train tracks were a masterwork of engineering in the day of their design. To come back down we decided on taxicab.
Now you should know the following about Indian taxicabs;
They don’t leave until all the seats are sold, 4 in the back , and 3 in the front PLUS the driver.
They are very old vehicles, and while they are doted on, they are the very definition of ‘beaters’.
No one on earth is more stingy with fossil fuel than an Indian cab driver.
They run the vehicles 7 days a week, 10 - 12 hr days.
You guessed it, all the way down from Darjeeling to Siliguri, engine disengaged. It was a little alarming at first, but no one else seemed to think it was anything special, so we just relaxed and enjoyed it.
Part of me believes that if it was really as damaging as you say, or produced no gas savings these fellows would have figured it out right smartly, and changed their ways.
I don’t know precisely. What I know is that some cars have fuel cut-off on deceleration. Maybe all do, I never had cause to research that. As to whether it occurs indefinitely so long as the accelerator pedal is not depressed, or only for a limited time, again that’s a detail I don’t know. But I feel confident in saying that fuel consumed while thus coasting is significantly less than it was with carburetors. The intricacy of fuel management strategies in modern cars is impressive. For example (unrelated but perhaps a helpful thing to know) many have a “clear flood” mode, where if the accelerator is held all the way down while cranking to start, fuel injection is disabled.
Possibly selfish and short-sighted of me, but I don’t care - I don’t pay for those parts - it’s a leased company car - I do have to buy my own diesel though, so I try to conserve that as carefully as possible. On a round trip to Winchester last weekend, I managed 64.8mpg
I’m sure that’s all true. But I was taking at face value the oft-repeated claim that a modern car uses less fuel when rolling down a hill in gear than in neutral, due to the fact that all modern cars shut off the fuel flow under those conditions. Now I’m wondering if that’s even true for all cars.
I’m not a experienced auto mechanic but, to the best of my knowledge no internal combustion engine use fuel in deceleration, i.e: in the accelerator isn’t depressed there is no fuel flowing, whether it’s injected or carburated. When descending a hill the RPM’s will go up due to the inertia of the vehicle and gravity, but there’s still no fuel use. If you’ve ever driven a car w/ one of those guages that monitor fuel mileage you may remember that the guage will go up dramatically when you’re decelerating and go down rapidly if you accelerate w/ your foot to the floor.
I used to freewheel, like some of you, thinking I was getting better mileage. Any gain I made was lost when I had to rplace the bearings on the input side of my differential. The mech. didn’t even ask, he told me I should stop coasting down hills, he had obviously seen this many times. I learned later, that most older hands were wise to this. It makes sense, those seals and bearings are designed to operate under torque, when you remove the torque they are spinning freely. That’s also why you shouldn’t tow a car w/ the drive wheels on the ground, same situation.
When I say hills, I’m talking 4-5-6-7% or more downgrades.
On brakes, the drums, or discs get hot, although discs are slower to do so as they get more airflow and cooling. When the discs/shoes get too hot they overheat the pads/shoe kiner, which quickly glazes and loses most of it’s friction qualities, they can even catch fire in extra extreme conditions. When the glazing starts, fading begins and it only takes a few seconds to lose all braking action. You the need a brake job, which will likely include new discs/drums, as they will warp from the heat.
BTW, concerning freewheeling. Old SAABs used to have a transmission freewheel. This device disengages the driveshaft from the wheels when they are outrunning the engine RPMs, effectively defeating engine braking. It was initially used to prevent the original 2 stroke models from seizing up, but they kept the feature through the early production of the model 99, long after they had gone to 4 stroke engines. IIRC, the freewheel could be selectively engaged, and they claimed it was a fuel economy measure. Apparently, SAAB didn’t consider driving downhill without the aid of engine braking a big problem. Note that Sweden has plenty of mountains.
Several U.S. auto cos. had the “freewheeling” feature. Chrysler and Hudson come to mind. I remember my dad talking about the dangers of the freewheeling feature of the Hudson Terraplane. Some overdrive transmissions still feature freewheeling in the overdrive gear.
Just because something was once popular, or some people do a certain thing, doesn’t make it a good idea. The accelerator is what controls the speed, and slowing, of a vehicle. Good drivers rarely use their brakes except when coming to a stop.
Negative. I know that neither of my fuel-injected cars shut off the fuel injectors while coasting. It’s one of the benefits of getting a chip. They’re both over 10 years old though, so it could be that all or almost all new cars do. They both use EEC-IV, which Ford has replaced with the much more complex EEC-V.
Yeah, my cars do do that. The diagnostics checks they can do is pretty amazing too. You can have it do a test where it cuts the fuel injectors one at a time while idling, calculate the RPM loss from each cylinder, and report if one of the cylinders is weak. It’s a surefire way to tell if one of the cylinders is missing.
Oh, and let me tell you about one of the dangers of rolling down a hill in neutral. If you exceed the speed at which the speed limiter kicks in, your engine will die (at least on some cars). I know this from experience. It’s not fun rolling down the Rocky Mountains at 115 MPH without power steering or power brakes. I doubt many people are stupid enough to coast that fast down a hill, but some cars have a much slower speed limiter, so I figured it was worth saying.
All engines use fuel when coasting, unless the computer is programmed to cut the fuel. The throttle plate has a small gap to let air through, and if air is going through the engine, fuel is too, again, unless the computer specifically cuts it off.
You’re not supposed to tow a car with an automatic transmission with the drive wheels on the ground without disconnecting the driveshaft for more than a certain distance. This is because the transmission isn’t lubricated when the engine isn’t running. I’m hesitant to believe that turning the differential without torque will mess up the bearings and seals.
I hate to post three times in a row, but I need to add something to what I said in my first post.
While my cars don’t cut the fuel upon coasting, they do close the idle air controller above a certain RPM (1200 I think) so it’s possible that less air and thus less fuel would flow through the engine compared to idling in neutral. There’s still some fuel being used though.
I don’t want to leave that, since it might mislead someone. You can get away w/ towing a manual transmission short distances, much more tham would be advisable w/ an automatic, but if you hook a manual tranmission car behind a motorhome and take off around he country, you’re going to have problems. I know, people do it, but they pay for it in the end.
if they brakes fail due to overheating there is no reason to think the emergency brake would also fail, you are talking about boiling liquid dumping bubbles into a closed system causeing failure in one case, in the other you yank a cable and it uses mechanical force to engage the same rear brakes.
on the other hand you would be a total moron to think you can safely stop a car from high speeds on a down hill while applying only the rear brakes, you are begging for a crash.
I would have to see some solid info to convice me you would get total failure in the rear brakes after overheating.
Overheating can cause drum brakes to warp, which reduces contact between the shoes and the drum and causes the brakes to fade. Many cars have rear drum brakes - the emergency brakes on these cars can fade even though the mechanism uses a cable instead of hydraulic fluid.
Another reason not to freewheel is that you can use engine braking and power to improve handling. I’d hate to go into a curve a little hot without being able to use the engine to help keep the wheels pointed in the right direction.