# Faster acceleration degrades fuel efficiency

## Does faster acceleration degrade fuel efficiency?

I've heard many times that harder acceleration results in poor fuel efficiency, but I have yet to come up with a solid explanation for this. That of course makes me question that advice. Of course, as you accelerate faster, more fuel will be pumped into your engine, but you will get to that great point faster, where fuel consumption is much lower. So is the payout worth it or not?

A few parameters are required for this question. If you stop at the traffic lights just 100 meters away, you are sure to face very poor fuel efficiency. I'm talking more about lights at the beginning of a long, open road, on which accelerating has no effect on possible braking.

I think the whole point of this question is the power per fuel use at different engine speeds. I've been thinking about it because my car has very low torque at low revs, but when it's over 3,000rpm it's great (old BMW 320i). Could it be that my car's engine is running more efficiently at 3,000 RPM and accelerating slightly harder than normal will improve fuel efficiency? Or am I dreaming?

Reply:

As you said, the crux of the problem has to do with the last amount of energy being extracted from one unit of fuel. You can think of this as your overall fuel efficiency.

Accelerating your vehicle from standstill to 100 km / h requires a fixed amount of energy based on the weight of the vehicle (excluding wind, friction and rolling resistance).

So you need to use your engine as efficiently as possible to generate that amount of energy. For example, you could just hit the accelerator and quickly produce the energy you need, but you have no way of knowing if this path is more efficient than building that energy over a longer period of time.

To know this, you need to know your engine. In particular, you need to know your Brake Specific Fuel Consumption (BSFC). The BSFC measures how much energy your engine is generating per unit of fuel (usually one gram of fuel). Your engine has different BSFC values ​​at different speeds and torques. You want to run your engine where it has the lowest BSFC (lowest fuel consumption per unit of energy produced). If you look at the BSFC table I linked you will find that the lowest BSFC for this engine is 206 at around 2100-2200 RPM, near peak torque. This means that this engine will run most efficiently in this operating range.

In general, engines run most efficiently near their peak torque with the throttle wide open and at the lowest possible RPM. This range is different for each engine and vehicle because all engines are different and each vehicle has different parasitic loads. You could probably calculate it for your vehicle if you're running the vehicle on a dynamometer to measure performance and then monitor fuel economy as well, then divide over the curve to see where the engine is most efficient.

Now that you know what operating conditions your vehicle is most efficient in, the difficulty of running your engine in those conditions while accelerating becomes ever greater. In the case of a transmission, this becomes difficult because the speed has to increase as the vehicle accelerates.

With a manual transmission using the example curve above, I would run the accelerator pedal at full throttle from 1500 to 2500 rpm and shift at 2500 rpm for each gear. I'm sure it is possible to calculate the exact switch points for maximum efficiency, but I'm not ready to go into that much effort.

With an automatic transmission this is very difficult because if you try to run the engine at full throttle the automatic transmission will extend your shift point into the high speed range and you will get very poor BSFC and efficiency. With an automatic transmission, you need to accelerate much more slowly so that the transmission shifts in the appropriate places so that your engine stays at a speed at which it can operate with the highest efficiency. You may be able to do this by pressing the accelerator pedal halfway, but you have to do it by your feet.

In conclusion, I would say the best way to accelerate is to accelerate as fast as possible at the lowest RPM.

Hope this explanation has helped a bit.

Wiki brake-specific fuel consumption

A certain amount of energy is required to accelerate a certain mass to a certain speed. So if you just look at these factors, you can rightly assume that how fast you accelerate that mass does not matter.

But that's not how the real world works. The problem with this is that internal combustion engines and the many things we associate them with are amazingly inefficient. There's a significant amount of friction. In order for your car to accelerate quickly, your engine needs to move faster. The faster a piston has to oscillate, the greater the force to which the bearings are exposed. The work required to pull air in and out of the cylinders through the valves is much greater, exponential, at high speeds. And the list goes on.

In addition, your car engine has to waste even more energy at high RPMs turning the generator, water pump, steering pump, etc.

When you put all of these factors together, it's pretty obvious why your engine is more efficient at lower revs.

Does faster acceleration degrade fuel efficiency?

Yes.

Of course, as you accelerate faster, more fuel will be pumped into your engine, but you will get to that great point faster, where fuel consumption is much lower. So is the payout worth it or not?

No.

This can easily be measured via the OBD II port. For example, my Accessport immediately shows the current fuel consumption. The combination of automatic dongle and smartphone should provide you with detailed information as well (but without the option of reprogramming your control unit). Their product tour says: "Fast Acceleration - Pressing hard on the accelerator pedal decreases fuel efficiency and wastes money." With a device like this one, you can measure exactly how much money you are losing on rabbit starts.

Accelerating requires more energy than driving in steady state and therefore requires more fuel. Accelerating quickly requires more energy than accelerating slowly. There is a super-linear relationship between acceleration rate and fuel consumption (more fuel consumption per meter / second ^ 2 at higher acceleration rates).

For a hands-on demonstration of how turtle acceleration can save fuel, I refer you to Jeremy Clarkson's demonstration in his epic ride from London to Edinburgh and back on a tank of fuel in a twin V8 turbo (scroll down to episode 4) of the season 4). It describes in detail how to avoid the wasted fuel of acceleration. Above all, he kept the speed below 1200 rpm.

These are measurable quantities. There is no justification for fuel efficiency when a rabbit starts at the traffic light.

Real life results

I have a 2005 Nissan Armada with Headersm CAI, custom exhaust and tuning. My wife drives like a grandma - acceleration is nice and slow, anticipation of braking gets an average of 12-14 mpg.

I drive this beast like I stole it everywhere - though still counting on the brakes and slows down - I usually pull an average of 14.8-15 mpg. I'm doing about 1-3 mpg better than her and I attribute it to accelerating as fast as I can to get to my cruising speed and then sitting there longer than her.

When you watch your scan display, it doesn't matter how much you accelerate the mpg until you reach cruising speed. The longer you sit in the bad MPG zone, the more gas you use. The scan meter doesn't lie and it doesn't give any real results either.

Test 3 full tanks and drive like you have a pair for your cruising speed. Find out what is better for your vehicle! I also have a Toyota Yaris that I use to drive 112 miles to and from work every day. Accelerating-like-I-have-a-couple-then-cruise got me 1.3 mpg more than slow acceleration over an average of 6 tanks.

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