So in my attempt to save gas, I find myself often coasting into stops or down hills, by either putting in the clutch or putting the car in neutral. I was talking to my dad about this and he says that I'm not saving any gas because if I'm engine braking I don't have the gas pedal in so there's no gas being used. This seems counter-intuitive to me. If the RPM is at say 5000 when I'm coasting down a hill or to a stop, its got to be using more gas than if I'm in neutral and coasting down a hill or to a stop at 800 RPM or whatever the MINI idles at. So what's the deal? I guess I'm a bit confused about exactly how an ICE works. I mean there still has to be combustion going on, even when the gas pedal isn't being pressed down, right? So at 5000 RPM you're going to be using more gas than at 800 even if you're not pressing in the gas, right?

 

Yeah, because the engine is still running, the injectors are working and it still needs the appropriate fuel to keep it running at whatever frequency (rpm) it is at. I like engine-braking often, but sometimes neutral is very tempting too, especially when you know it can coast for a couple miles down a certain stretch of road.

 

I would say you're correct in more fuel used at 5K than 800 RPM, but using this method to save gas is (mildly) dangerous and relatively futile. Better to have the car in gear (a higher one - use the brakes to slow down) so you can accelerate/avoid as necessary.
You'll save much more by taking your bike to the store every couple of weeks instead.

 

Your instincts are totally correct. Engines are for going and brakes are for stopping. You can save a lot of gas and a LOT of engine by not using your engine as a brake. It is not designed for that purpose. Further, it is a lot cheaper and easier to replace a set of worn brake pads than a set of worn or broke piston rings and damaged valves.

People think that it is cool and proper to emulate race drivers......it ain't...race drivers rebuild their engines every 1000 miles (at best).......

Also, an engine will use a LOT more fuel at 5000 RPM than at a typical idle of 1000 RPM.

 

Coast in nuetral though not with the clutch depressed,you may tend to ride on it

 

your dad is correct. At high RPM, if you let off of the throttle the engine control will cut fuel. As the RPM decreases due to engine braking, the engine control unit will start turning on the fuel injectors to prevent stalling. If you want to save gas on decels, leave it in the gear you are already and leave it in gear until the RPM drops to around 1800-2000 RPM and depress the clutch and put it in neutral for the remainder of the stop.

Experiment: If you are in a lower gear like 1st or 2nd, try leaving the clutch in until 1000 RPM coming from 4000 or so. As you decel, you will notice a slight jerk around 1800 RPM, that's the Engine adding fuel again.

I don't recommend going gear to gear until you come to a stop as it only wears your clutch, but leaving it in gear will save your brakes, gas, and is no additional wear on the engine.

 

Many of us complain about the poor mpg we get but we drive with higher revs.
You will burn more fuel at 5000 rpm than at 800 rpm.

At the same time you want to be safe and not wear out other things in your efforts to save gas and be efficient. There are better ways to save fuel than to coast downhill in neutral (it's not safe since you cannot use throttle to get out of unforeseen trouble).

Keep a constant velocity (don't overly use your brake or accelerate too hard only to slow down). Keep your speed down in the 60 mph range than above 70. 45 to 50 is good if the traffic allows. Stay away from idling in traffic, low revs and going nowhere fast. Use your cruise control if you can (if it is safe to do so).

From a stop speed up smoothly and gently and only so fast as the traffic will allow. If you can avoid using your brakes most of the day you are doing pretty good. It really depends where you drive and what are the roads like- congested or not.

 

Speed----MPG (100% highway miles continuous)
--70-----38
--75-----36
--80-----35

Interesting how the fuel economy goes down with increasing speed huh?

 

On carburetted engines fuel usage is determined by the airflow through the venturi of the carburator. Taking your foot off the pedal at 5000 rpm creates a high vacuum condition in the intake and fuel is still being used, but not at the same rate as if the throttle were open. With the throttle closed there is insufficient air for complete combustion and the excess fuel washes the protective coating of oil off of the cylinder walls increasing wear. It also leaks down around the rings and dilutes the oil in the crankcase.
On electronically fuel injected engines, like our MINI, the ecu turns off the fuel injectors when the throttle is closed and will keep them off until the throttle is reopened or the engine reaches a preset stall speed when they are turned on again to keep the engine running, producing that little lurch you feel just before you disengage the clutch when coming to a stop. Road & Track had an explanation of this in one of their recent issues.
Yes, brakes are cheaper to replace, but brakes fade with repeated use and temperature. Try driving down a mountain using only your brakes and you will see what I mean. A combination of engine braking and friction braking will produce the least wear and tear. Incidentally, the greatest engine wear occurs at start-up, when there is little oil on the cylinder walls and bearings. It usually takes about 5-10 seconds before the oil starts to circulate through the engine, and longer before it reaches the correct operating temperature. So it is important not to rev the engine at start-up, or even until the engine is warmed up.

 

Really? So the injectors go off.. and basically the engine isn't working? err.. combusting? And the oil, is that still circulating? My interest is really knowing HOW bad it is to engine brake. At least that explains the engine "hesitation" when the RPM's are falling and pass exactly at 1900rpm's everytime, although it wasn't noticeable until the pulley was installed.

 

Thats correct, there is no combustion as you coast down. Yes, the engine is still turning over, because the wheels, through the transmission and driveline are turning it over. Is oil still circulating? Yes, the MINI like most engines has a mechanical oil pump geared to the crankshaft of the engine. Instead of the engine driving the wheels, the wheels are driving the engine. It is the resistance of the engine, the power needed to overcome the compression and friction, that produces "engine braking". Put your car in neutral, with the engine off, and try to push it, you'll see what I mean. Thats how you push start a car. You push it in neutral to get it rolling, then release clutch with the car in gear and the ignition on.The momentum of the car rolling turns the engine over and hopefully it starts. On carburreted cars if you closed the throttle at high engine speed and let the car coast, air would still flow through the carburator, though greatly diminished in volume, drawing some fuel into the engine and causing it to fire or combust intermittently. This would make that popping sound that you can hear in old racing films. Occassionaly the driver would rev the engine while braking (heel and toe) to downshift and clear the intake of excess fuel. Lots of fun!

 

Its amazing to hear people talk about a subject that they seem to have no knowledge of. Engine braking is a part of driving a stick shift vehicle. Always has been - always will be. Those big rig drivers are not coasting in neutral going down the Donner Pass relying on their brakes. High performance cars, especially, are engineered and designed to use engine braking as part of their high performance capabilities. Anyone who tells you that engine braking will cause gas in your oil, or use the same amount of gas as powered driving is dillusional. Truck drivers have known this from day 1 and modern cars have electronic brains to avoid any possibility of problems. Your injectors do not fire based just on engine RPM - it is a complicated calculation of rpm, load, throttle position, and other factors.

In the old days, when your carbourated British car was fouling its plugs, you were told to go out and go 70 mph and then turn off the ignition while driving and pump the accelerator. The flow of unburned fuel hitting the hot plugs caused the dirt to chunk off, cleaning the plugs. If you do not pump the accelerator, you are just engine breaking. Fuel injection cars are very sparing in their fuel use - if you are not accelerating or stalling or under load, they do not want to squirt fuel into those cylinders - thats how they keep their mileage ratings as high as possible. Engine braking feels exactly the same as turning the ignition off with the car in gear - only in the old days your steering wheel did not lock!

Engine breaking will in no way harm your engine, and if the wheels are driving the engine, the brain will not waste fuel because it is under NEGATIVE LOAD. If the brain in your car is not smart enough to figure that out, we are all in trouble.

 

Have you tried 50 or 55? I am guessing it will be in the 50's.......

 

[QUOTE=JoeDentist]Its amazing to hear people talk about a subject that they seem to have no knowledge of. Engine braking is a part of driving a stick shift vehicle. Always has been - always will be. Those big rig drivers are not coasting in neutral going down the Donner Pass relying on their brakes.


er....with respect.....ever hear of something called a Jake Brake??? You just might want to check into that.....
Also with respect.......what has always been part of properly driving a stick shift, and still is, is to use match the engine revs to the gearbox revs so the engine will be in the most appropiate rev range for the gear selected once the clutch is released. It is not proper to use your engine as a brake, or maybe I should say it is no that smart on a street car. Not if you want to keep the car for a loooong time. If you're gonna trade it in a couple of years, hell, don't bother changing the oil in it....

 

I don't have any Nitrous in my garage, so I don't think I'm dillusional when I say that unburned gasoline can get in your oil via worn rings or valve seals, it's called "blow-by". As I said in my post, in carburreted cars, unburned gasoline can wash the oil coating in the cylinders and cause excessive wear to the cylinder walls and piston rings. This can lead to dilution of the oil by the gasoline "blowing by" the piston rings. This is NOT a problem in electronically fuel injected cars as the fuel injectors are turned off when the throttle is closed, except at idle.
Do I use engine braking? Of course! I don't see how you can drive a car without using some engine braking. The problems arise when you use excessive engine braking or no engine braking at all. You have to balance the two.
Most "big rigs" are diesel, and are fuel injected. Diesel fuel provides more lubrication than gasoline. Going down the Donner Pass or any long steep incline is most likely going to require engine braking, but that doesn't mean its good for the engine, it's just safer than relying on brakes which can overheat and fade. However they must be careful not to over-rev the engines and damage them, so they alternate between engine braking and regular braking.
I've never turned off the ignition and pumped the throttle to clear the plugs,
most of the time I took it back to my shop to clean the plugs and adjust the carburators, but I have retarded the ignition timing and caused the engine to backfire to remove carbon build-up, that is, on cars that had a manual spark adjustor.
I will find the Road & Track article I mentioned in my previous post and give you the date and page.

 

Trippy,

One qestion.. on a 5600 mile trip at 70 MPH it would take 147 gal of gas
same trip at 80 MPH it would take 160 gal of gas
at $2.25 per it would be $330.75 at 70 MPH and $360.00 at 80 MPH
saving $29.25 by going 70 over going 80 (or is that under).
Thereby taking 10 hours longer on this trip and staying one more night in the motel. This would cost more in the long run. Now do you think the nice policeman would buy this as a way of saving money?

Earl

 

Engine braking in our cars absolutley saves gas. They are fuel injected and computer controlled and when you take your foot off the gas the cpu tells the injectors to stop allowing gas to pass "no pun intended".

During engine braking the momentum of the car turns the engine and there is no need for fuel and you are getting infinite MPGs.

If you coast in neutral, the the engine needs gas to idle and therefore you are comsuming gas.

You can witness this yourself by using the computer average MPG or the test mode #4 to see instantanious MPG. (See thread http://www.mini2.com/forum/showthrea...&threadid=3531 )

My experience shows the CPU cuts off fuel until RPMs drop to around 2000.

 

You are absolutely correct! Many people keep quoting how on their carb would cause blow-by. Fuel injection does not add fuel and therefore no blow-by. If fuel-injected cars continued to put gas in, it would not burn well and cause them not to meet emissions.

 

In many states it is illegal to place a car in neutral while moving on a public roadway.

M

 

As we are on this great subject. Can anybody explain why I never felt that lurch at 1800.. 1850.. I think it's 1850 rpm's when my car was stock. Well, I didn't feel it with the intake mod. But the moment I drove off with the 15% pulley was when I noticed it immediately, like a weird sideeffect of the pulley. And I didn't know what that lurch was until this thread, eek! Shows how used to it I got, I just had let it go. Now, why would this lurch be a lot more prevalent from the pulley? I doubt it's a problem, but just wondering.

Oh yeah, I wanted to hear the difference, for the first time. Just letting the engine and car slow down slowly and hearing the exhaust, the moment it adds fuel again at 1850rpm's, you can hear the exhaust gets immediately burbulier, if that's a word.

 

Here's something I have knowledge of: When you begin your post by telling the other posters how dumb they are you toss any credibility you might otherwise have.

R.E.

 

i agree with this

 

you are comparing apples and oranges. diesel engines regulate fuel to control speed, gasoline engines regulate air, when modern EFI engines brake-as explained above- fuel is not consumed, and the pistons just pump air

The jake brake is a fuel pump cut-out switch, it is just a more efficient form of engine braking for diesels.

Coasting (either in neutral or with the clutch depressed) is not a good idea, your control of the vehicle is reduced, you excessively wear on the clutch or the trans (when moving and in neutral, the rotating parts connected to the wheels rotate at a different speed than those connected to the engine-not good for synchro rings) with the clutch depressed, you also wear the throw-out bearing (release bearing) and the springs of the clutch-release plate (pressure plate).

 

I have a stock MCS, and I never noticed that little bump on deceleration until I read about it on the stumble thread.Now I notice it if I let the engine revs get down that low while still in gear. I usually push the clutch in a little earlier than that. Could be that because you engine is making slightly more power at a given rpm due to the smaller pulley the bump is more prominent or could be now that you know about it you notice it more. I've been going through my Road&Track magazines, searching the Technical Tidbits and Technical Correspondence articles for the article on fuel injection and I've come across quite a few references against engine braking but I just don't see how you can drive a car without doing it. Like anything done to excess it can be harmful but in normal driving I just don't see it.

 

Keep in mind that coasting down a hill in neutral is fine, but applying brakes down a long hill can cause more than brake fade. There is the possibility that long braking times will not only cause the pads/rotor to heat up, but also bring the brake fluid to a boil, which will severly impact your brakes more permanently than brake fade will.

I use a mixture of both depending on where I'm at and the length of the hill. I'm more in agreement with the fact that the engine is using no more than idle fuel pressure at the higher engine braking RPM. I would think that if that is the case then you would be using no more fuel than coasting in neutral. I would think that your engine is under more pressure though, after all, spinning at 5000 RPM is harder to do than spinning at 800.