Hello there,
during a routine chat with some friends, the topic came about of braking - reducing the speed - of a car using its engine. Naturally, there were those for and those against. Aside from the easier technical issues, as that using the engine spares your brakepads and oil (which is actually very important in trucks), the security issues, like the fact that maintaining traction in the wheels prevents loss of control, we could not decide upon engine wear and fuel consumption. Please note, this is the ideal driver case - no gas-brake-gas driving, no rally driving, etc, etc. Also, let's leave noise out of the issue for now.
Now, according to some resources on the net, engine wear is not a problem. Fuel consumption is a different issue: assume you're driving 60 km/h, then you disengaged the motor - it's idling. Next you press the brakes, and gently slow down to 20 km/h. The amount of fuel is just the time multiplied by the idling consumption.
What happens if you shift to a lower gear instead of disengaging completely? Perhaps you'll slow down to 20 faster - but what was the instantaneous consumption?
Does anyone have experience on this? Indeed, on a curve I usually brake, shift to a lower gear, and do the turn safely. But on a straight road I prefer to use the brakes to bring the car to a proper speed, then engage a suitable gear. Am I really gaining in fuel here?
Renato
I'm a bit tired, but it seems you are looking at 3 situations.
1. Shifting to neutral and apply brakes.
2. Down shifting and brakes.
3. Stay in gear and brake.
I think option 2 would use the most gas due to increased RPMs. Option one seems to be the most fuel efficient. Another, trick is to watch the lights ahead and simply slow down ahead of time so that you don't have to come to a stop at all. 
Of course, this pisses off some folk who don't seem to realize they aren't going anywhere anyhow until the light is green.
Another issue that has come up in talks is whether you are saving your brake pads at the expense of your clutch. Anyone ahve thoughts on that? Ok, i have limited knowledge as far as this goes, but from what I know, #1 is you best option, since your brakes are more suited to stop your car, as compared to your clutch ^_^
Staying in gear and braking is also fine, but is less fuel conscience than going into neutral.
| HoboPelican wrote: |
| Another issue that has come up in talks is whether you are saving your brake pads at the expense of your clutch. |
That's true, and a clutch is a lot more expensive to replace than brakes. But the car is wearing out all the time you use it so is it more important to preserve the car or ride in safety and comfort?
I think that the method of changing down gears and using the engine to brake is taught to new drivers because it's safer. If the car in front of you suddenly stops, something it's more likely to do while traffic is slowing down, then you have your brakes and your engine helping you to stop.
Another thing is comfort. Using the engine to brake results in a much smoother decelleration and stop. Try stopping just using the brakes, in neutral, and you impart more G-force to the passengers causing them to lurch forward in their seats.
Professional chauffers are taught to drive in a way that doesn't involve any harsh acceleration or decelleration. They can drive with an egg in an eggcup in the footwell without it falling out. This uses more fuel but is a more pleasant experience. You have to pay for comfort.
A friend of mine in High School had to drive with her father holding a full cup of water and wasn't allowed to spill a drop. That was a little extreme, I think. | HoboPelican wrote: |
| A friend of mine in High School had to drive with her father holding a full cup of water and wasn't allowed to spill a drop. That was a little extreme, I think. |
Definitely extreme . I guess he didn't want her borrowing his car.Well, I did read in an auto magazine that one of the most traumatic experiences for a family man was lending the car to his kid for the first time. I know it was such for my father - he stayed up till I came back, then went to check everything was in place - and kept doing it for at least 4 months..
Indeed, I had completely forgotten about the clutch. It's understandable, I suppose, since I jumped from a bycicle to the car.
Fuel efficiency is really something on our minds here in Europe. From what I gather, the trend is finally catching on in the States. In any case, the issue is harder than simply teaching parsimony to drivers. If we could find a way to get old, inefficient cars to consume less, it would be bliss.
Then there's the alternative fuels.. strange as it may seem, some students at the university here adapted their diesel engine to run with rape seed oil. No, not biodiesel - oil, direct from food store. Drawbacks are its inability to start under 10 C, the amount of water it absorbs from air, and the peculiar smell of fries that it leaves behind.. It is difficult to find authoritative data on these engines, but afficionados say it's cleaner, and sucks about 5 litres/100 km. They do concede that a certain amount of performance is lost with this fuel, which is rather low-octane. Anyway, they're allowed around here - and here everything's forbidden until it's allowed.
| HoboPelican wrote: |
I'm a bit tired, but it seems you are looking at 3 situations.
1. Shifting to neutral and apply brakes.
2. Down shifting and brakes.
3. Stay in gear and brake.
I think option 2 would use the most gas due to increased RPMs. |
No, because (assuming that you get off from the gas pedal) the engine doesn't get more fuel than it would get in neutral. The energy from the high RPM comes from the car's mass and inertia, not from burning of fuel. Thus the speed is being "eaten up" by the engine.
| HoboPelican wrote: |
| Another, trick is to watch the lights ahead and simply slow down ahead of time so that you don't have to come to a stop at all. Of course, this pisses off some folk who don't seem to realize they aren't going anywhere anyhow until the light is green. |
Yes, I do it this way, and yes, someone will never get why I don't accelerate like a fool... until he brakes at red light, like a fool. 
This is everyone's own decision. I usually just step off from the gas pedal, and when the RPM drops under ~1/3 of the max, I shift down. Quite elegant, I think...On the wear of the clutch...
The clutch is really like a break, except that, if you shift well the discs are spinning at nearly the same speed as they engage. This means that the friction and hence the wear between the plates is a lot less than when you apply the wheel brakes. So for down shifting (well), the wear argument does not apply.
Riding the clutch to get going on an uphill, that is a different matter. Here you are relying on lubricated friction in your clutch to control the torque to the wheels. That will do some damage to clutch lubricant and plates.
| Nyizsa wrote: |
No, because (assuming that you get off from the gas pedal) the engine doesn't get more fuel than it would get in neutral. The energy from the high RPM comes from the car's mass and inertia, not from burning of fuel. Thus the speed is being "eaten up" by the engine. |
again, I am no expert, but would the higher revs still pull fuel through the carb? This may be only with a carb system, I know even less about injection. My understanding was that the fuel mix was controlled by air flow. Accel fuel is tied to the pedal, but as long as air is being sucked in, so is fuel.
So, how wrong am I?
Yes, the amount of fuel the engine gets is controlled by air flow. And air flow is controlled by the gas pedal. So if you release the gas pedal, the "butterfly-valve" (or whatever it is called in English) closes, letting no air in, thus no fuel will be pulled. (A minimal amount will get there anyway, because if the valve is closed, a bypass opens in order not to stop the engine in neutral.) So the gas pedal controls air flow, not fuel intake. (Injectors are a bit different, but the same rule apply.)
If you can, check a car with an "actual consumption meter" in it! When you let it go by itself, it will measure 0.2-0.5 l/100 km.
busses and big trucks have a nice apparatus for braking using the engine (together with their brakes): It's called a Retarder.
There are two different retarders: one works on the principle of a rotor rotating in fluid. The more fluid added, the more friction the rotor meets and the heavier the brake from the retarder.
The other retarder, often used in public transportation busses, works similar like a dynamo. A large magnet is rotating in between spools and this finds heavy resistance, slowing the vehicle down.
Retarders are not used for stop-braking, but only for high speed braking. Thats's because they are uncontroled most of the time. Friction brakes are used for stop braking (the normal brakes you see on a car, motor or cycle, drums or discs.)
Would be a pretty nice feat, a very fast family car (like a maserati or bmw M5, with a retarder..., braking as fast as accelarating. But retarders are heavy and increase fuell load and accelaration. So most of the time they're only used in public transportation (big busses) or the commercial trucks.
| Nyizsa wrote: |
Yes, the amount of fuel the engine gets is controlled by air flow. And air flow is controlled by the gas pedal. So if you release the gas pedal, the "butterfly-valve" (or whatever it is called in English) closes, letting no air in, thus no fuel will be pulled. (A minimal amount will get there anyway, because if the valve is closed, a bypass opens in order not to stop the engine in neutral.) So the gas pedal controls air flow, not fuel intake. (Injectors are a bit different, but the same rule apply.)
If you can, check a car with an "actual consumption meter" in it! When you let it go by itself, it will measure 0.2-0.5 l/100 km. |
Did some reasearch and it seems that on newer cars with fuel injection or carbs with fuel cut-off solenoid, you are correct. The cut off solenoid is needed since anytime the rpms increase, air flow increases.
Typical quote from research-
"Temporarily shifting to neutral on a sufficiently lengthy downhill grade will dramatically increase mileage for carburetor cars, while cars with fuel injection - or carburetor cars with a fuel cut-off solenoid - will benefit more from the fuel cutoff when the car is left in gear." | rdrs wrote: |
the security issues, like the fact that maintaining traction in the wheels prevents loss of control, |
If you have ABS, (nearly all modern cars do) this is not a problem.
| rdrs wrote: |
we could not decide upon engine wear and fuel consumption.
|
The only way engine braking is more fuel-efficient is that is cuases you to slow down gradualy, which you can do with the brakes. | rdrs wrote: |
Does anyone have experience on this? Indeed, on a curve I usually brake, shift to a lower gear, and do the turn safely. But on a straight road I prefer to use the brakes to bring the car to a proper speed, then engage a suitable gear. Am I really gaining in fuel here?
|
Just give your car a tune-up, and make sure the tires are all perfectly inflated, and you'll save more gas that way than you could by altering your driving in such small ways. | ocalhoun wrote: |
If you have ABS, (nearly all modern cars do) this is not a problem. |
ABS is just a controlled brake system. You still have a wheel that turns, and a brake pad that does not. Energy dissipation is directly related to the amount of wear on the pads and disc.
| ocalhoun wrote: |
The only way engine braking is more fuel-efficient is that is cuases you to slow down gradualy, which you can do with the brakes. |
It has nothing to do with a gradual slow-down. Unlike brakes, the disks in the clutch engage when they are nearly at the same speed, minimizing wear. The breaking comes from the vacuum in the cylinders, working somewhat like a pneumatic/hydrolic damper. Dissipated energy probably heats the engine block, oil and cooling liquid.
| ocalhoun wrote: |
Just give your car a tune-up, and make sure the tires are all perfectly inflated, and you'll save more gas that way than you could by altering your driving in such small ways. |
Always a good idea. | withaar wrote: |
| ocalhoun wrote: |
If you have ABS, (nearly all modern cars do) this is not a problem. |
ABS is just a controlled brake system. You still have a wheel that turns, and a brake pad that does not. Energy dissipation is directly related to the amount of wear on the pads and disc.
|
The problem that I was refering to was loosing control while slowing down. If you have ABS and still worry about loosing control during a normal stop, you need a mechanic.
