Coasting improve gas mileage?

jerry_njOctober 24, 2009

Here let me start by saying I'm not asking if it is smart or even legal to operate a car in neutral... but I have had an argument along this line.

Putting a car in neutral when going down a hill will increase the mpg more than letting the engine operate as a "brake", i.e., be driven at by the wheels - throttle "closed".

The argument I have heard (besides coasting being illegal and unsafe) is that the engine in neutral is burning fuel to maintain idle at about 750 rpm or so while the engaged engine has the fuel shut down, so even though the rpm may be 3,000, it is using less fuel. I say none sense, in both cases the throttle is saying minimum fuel and in one the engine is running 4 times as fast as the other (coasting/idle) case. Oh yes, the car costing may also go another 1/4 mile or more past the downgrade before the engine has to be again loaded with moving the car.

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"Depends" Jerry.

Decel fuel cut-off is a very common strategy that even existed in carbureted cars (First one I can think of was Ford Escort 1980). When all we had were carbs, decel caused an extremely rich air/fuel ratio and wasted a lot of fuel while simply releasing most of it to the atmosphere right through the engines tailpipe. So they added fuel cut solenoids to shut the idle circuit down during deceleration. Once we had fuel injection interrupting the injector circuit operation was easy to reduce decel fuel waste, and that is standard across the board with every manufacturer. It's use allows for better catalytic convertor operation as well. Very rich fuel ratios deplete the catalyst of stored O2 and in effect turn them off. Combine that with an additional clutch in the transmission that allows for reduced, if not eliminated engine braking when in overdrive and the engineers are attempting to out gain the benefits of even coasting in neutral and not from just a fuel mileage consideration, but an emissions consideration as well.

Today's hybrid vehicles are noted for not having any engine braking at all on certain models. In fact once they go into regenerative braking, and if the battery is sufficiently charged they will rely totally on the mechanical brakes to slow the car down unless "B mode" is used. "B mode" is known as mountain braking and uses the electric motors to fight each other to produce the effect of engine braking.

You have to study engine control dynamics to see exactly what compression numbers occur during different engine operating conditions, but as far as the argument that you are burning fuel during a strong decel, "There isn't enough compression to properly burn any fuel that gets into the cylinders" once the manifold vacuum reaches a high enough value. So most of the fuel getting to the cylinders simply gets pumped to the exhaust, which quite often would resulted in a pretty loud backfire. Use the lack of such an occurrance, that cars don't back fire out the exhaust like they used to as part of the proof.

    Bookmark   October 25, 2009 at 8:59AM
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Thanks John, very interesting and it raises another operational question, intended engine braking. I drive a manual transmission most (almost all) of the time. I still downshift to slow down. Is it possible that the modern car, I've got a 2009 Suzuki SX4 with a 5 speed, the fuel cutoff reduces the intended engine braking? I do note a significant difference in deceleration between 5th and 4th gear, I assume that is due to the difference in rpm:ground-speed, and not a different cut-off strategy.

While your description answers a lot, it still leaves unanswered, as you say "depends", if coasting gives more mpg. I did takeaway the sense that coasting may be cleaner than letting the engine operate in cut-off mode.

    Bookmark   October 25, 2009 at 9:55AM
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You have to leave room for varying operational strategies as you attempt to understand everything on this single subject.

Older carbureted car, without a decel fuel cut strategy. Coasting is cleaner, and more fuel efficient than just a closed throttle decel.

Your Suzuki, will be almost just as clean during decel in almost any gear above 15mph as would coasting. One could argue decel would in fact be "cleaner" than coasting because your still using fuel to maintain an idle while coasting, where the decel fuel cut is simply allowing the engine to turn over and only pump air through with no dispensing of fuel at all. Any difference in engine braking between gears would be the result of mechanical advantage from the drive train to the engine. The lower the gear 5th-4th-3rd, the greater the engine braking effect.

Carrying some points to an extreme sometimes is required to prove concept. Imagine a 1000 mile hill where your car would coast at a steady 60mph if allowed to. Which way would use more fuel? Allowing the engine to idle the whole time and coast in neutral, or allowing the system to make use of the designed fuel cut strategy?

Take a Prius for example. There is no engine braking, period. When you lift the throttle, the HV PCM (High Voltage Powertrain Control Module) can easily decide that the ICE (Internal Combustion Engine) does not need to be running and will shut it off. Here you have a full coast, no idle condition by design for the purposes of reducing tailpipe emissions and increasing fuel economy both at the same time.

    Bookmark   October 25, 2009 at 11:47AM
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Thanks, I had no idea about the extra clutch function in manual transmissions. I recall the first time I had a hint of that was when we purchased a 2004 Subaru Forester. The salesman referred to 5th gear as "overdrive". I took that as just another name for a gear that I'd use all the time, even at 30 mph on level ground. I think the Suzuki also refers to 5th gear as "overdrive". That said, I can say all 5 speed manuual transmissions I've driven have very little engine braking in 5th gear.

A good test for me will be to go down a familiar Interstate hill I have used many times and see how far the car will go in deceleration compared to what it has done in neutral. If it is close to the same then I'd be convinced there is little to no engine braking goins on and I'm wasting a clutch operation and perhaps some fuel, the idle speed consumption on the down hill run. Now if coasting gets me another quarter mile or more there is an argument to be made that coasting may be using less fuel. The run in question is almost 2 miles of downhill to get two mile if I start at 70 mph and engage the engine at 55 mph. This is not a "game" for someone in a hurry.

    Bookmark   October 25, 2009 at 7:43PM
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It's not the manuals that have the "extra" clutch, that is something that has been added to the automatics. Your maunal transmission has at least a single overdrive ratio, some are actually a double overdrive today (4th and 5th). Just like you don't accelerate well in the higher gears, hence the need to downshift to climb a grade or pass another car, your engine loses mechanical advantage torquewise, but by slowing the engines revolutions per mile you gain fuel mileage up to a certain point.

BTW don't exclude final drive ratio's in this equation as well. They are an important factor in determining the final crankshaft rotations ratio per mile.

    Bookmark   October 25, 2009 at 8:07PM
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About Overdrive

In ordinary manual transmissions, the input to output ration for top gear is 1 to 1, i.e., the speed of the output shaft is the same as the input shaft. However, if gearing is provided for the top gear such that the output shaft rotates faster than the input shaft, it is said to have "overdrive".

To coast or not coast

When coasting down hill, there is a small difference in fuel comsumption between a coasting/idling engine and one that is forced to spin faster than its idle (closed throttle) setting. Here's why.

The fuel rate is not a direct function of engine speed, although, engine speed is an indirect factor. For a carbureated engine, the fuel rate is determined by throttle opening and manifold vacuum. Forcing the engine to spin faster than idle increases manifold vacuum some which inreases the flow rate of the fuel/air mixture. The increase in vacuum is not a linear function of forced increase of engine speed because at idle, it is already operating at near maximum vacuum. For example, suppose the idle speed is 700 rpm. Forcing the engine to 1000 rpm will increase the vacuum by some amount, but adding another 300 rpm (1300 rpm) will not double the vacuum increment. There is a maximum vacuum that an engine can produce principally determined by its compression ratio. (Think of a piston engine as an 'air pump'.) This maximum value is approached gradually with increased forced engine speed.

Now, consider a modern auto engine having fuel injectors with electronic injector control - all bets are off. It all depends on the control program. If the engine controller shuts off fuel supply on decel, then the fuel comsumption would be less than an idling engine, because an idling engine must be fed some fuel to keep it running. See john_g's remarks above.

    Bookmark   October 25, 2009 at 10:43PM
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Good discussion, thanks John and jemdandy.

Given there is no free-wheel going on, as in the case of hybrids, then the load of turning the fuel-starved engine over will slow down the vehicle relative to a vehicle running in neutral. So, as I have sensed when driving down a familiar hill, the car in neutral goes further before power has to be applied to maintain a desired forward speed.

The real good news is either way the car is getting great mileage during the downhill run, helps compensate for the poor mileage coming up the other side.

Given the computational resources of new vehicles I suppose it is reasonable to see the controller being able to detect the off throttle + over some minimum mph + some delay(?) to be sufficient to cut the fuel off. However it works, I never feel the engine regaining power, it works very smoothly.

The new Suzuki I purchased has (I am told) an electric interface between the foot peddle and the injectors, not mechanical, or even hydralic, linkage. In the case of this car it introduces a slight, perceptible, delay in throttle down, noticed when shifting a manual transmission. If one does the simultaneous off throttle, clutch in, there is a noticeable rpm "jump". This doesn't do any harm, I think, but I don't like it and am developing the off throttle then clutch in. I think it will become automatic, but when I drive another vehicle, e.g., my 2005 Chevy Colorado, the shift difference is noticed again. I assume what I have been told about the throttle control is right, "drive by wire" rather than linkage.

    Bookmark   October 26, 2009 at 8:11PM
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Coasting uses more fuel in a modern car. Have a look at the experiment Top Gear did driving a big Audi from London to Edinburgh and back on one tank. for lots of tips on getting as many miles as possible from each gallon have a gander at their website and the library of episodes to select this particular challenge.

Here is a link that might be useful: TopGear

    Bookmark   November 2, 2009 at 7:25PM
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