Though it may seem counterintuitive to many, the frequency and speed of your brake application (as opposed to your accelerator) will have the biggest impact on fuel consumption particularly for a driver that mostly drives in city driving situations. In the US, the EPA applies a weighting of 55% for miles in the city MPG drive cycle and 45% for highway in calculating the overall expected MPG value. Have you ever thought hard about why the city MPG is lower than the highway MPG? You probably reasoned it being due to stop and go driving without giving too much thought beyond that. If you think a little more deeply shouldn’t city MPG be higher, after all average speeds are lower than on the highway which means lower aerodynamic drag, lower rolling resistance, etc. so why is city mpg for most non-hybrid conventional cars higher?
BRAKING
It’s really because most of the excess momentum (kinetic energy), that was generated after reaching your desired speed, is turned into wasted heat energy by braking. When I say excess momentum I mean maintaining your speed due to keeping the accelerator depressed for longer than necessary when the traffic ahead is stopped or moving slowly, or when there is a red light or stop sign ahead and then braking at the last second. Or worse, accelerating when it is clear that a slow down or stop is imminent, such as accelerating to a red light or a stop sign which then requires rapid braking. The fuel used to maintain or increase one’s speed prior to using one’s brakes to stop or slow down repeatedly is the fuel that wasted which almost fully accounts for why the city MPG is lower than the highway MPG. If one calculates the tremendous amount of heat energy given off by the brakes coupled with most engines not being in their efficient operating range for a large portion of the time over the duration of a trip (converting fuel energy into motion), it becomes readily apparent why the city MPG easily falls below the highway MPG. Add in the fact that the sooner one reaches the red light, after stopping quickly, the longer one will be idling the engine waiting at the red light, producing no forward motion for the fuel consumed. Every bit of the energy expended by the brakes came from the fuel that was used to accelerate the vehicle up to the speed it was travelling at just prior to braking. If you brake a 3500 lb car from 60 mph to 0 mph in 5 seconds (which is not that an unusual deceleration rate), your brakes are expending energy at a rate of up to 200 horsepower (~150 kW) depending on road/tire traction conditions. (over 550 kJoules will be expended). Approaching an imminent stop or slowdown at excess speed and then braking rapidly may be fun for a while (you experienced more of the car’s capabilities) but you pay later (sooner) at the pump and for replacement pads at the brake shop or dealer.
ACCELERATING
Also counterintuitively, many people mistakenly think that accelerating gently will improve their fuel economy, this is also not the case. In modern cars you will usually burn more fuel accelerating up to 40 mph in 16 seconds than you will accelerating to 40 mph in 8 seconds (that’s not a typo). Let’s think of it this way, if you’re taking 16 seconds to accelerate to 40 mph, you’re consuming fuel at a lower rate but you’re doing so for twice as long which would make it awash whether you accelerated slowly or quickly if engines were equally efficient at all RPMs, however engines have an operating RPM where they produce the most work (acceleration) for the amount of fuel consumed (look up BSFC) This is often, though not always, around the RPM that produces maximum torque. Maximum torque is the point where the engine accelerates quickest. So in any given gear you get pushed most into the back of your seat at the engine RPM that the maximum torque is produced (while in that gear). So for example, if you car produces maximum torque at 3500 RPM and your in 3rd gear, any engine speed less than or greater than 3500 RPM produces less acceleration. So to accelerate most efficiently you should accelerate in an engine speed range that centers around the RPM of maximum torque. So in our example one would accelerate until about 4000 RPM and then upshift (change gear) which would bring the engine back down to about 3000 RPM depending of course on the specific transmission. This RPM range would be the “sweet spot” for accelerating for maximum economy. In an automatic transmission, you depress the accelerator such that the RPM “hangs” around the optimum torque RPM. For many modern cars this may mean you have to accelerate at a higher rate than expected for maximum economy.
To bring the above 2 paragraphs together (whose strategies at first glance may seem at odds with each other, i.e. don’t brake, don’t accelerate, accelerate quickly) into one sentence, one must accelerate efficiently (at peak torque RPM, which often means quickly) to the speed such that one will subsequently not have to use the brakes (or least minimize their use). For city driving, optimizing the braking will typically produce the bulk of the efficiency improvements and optimizing the acceleration much of the rest. That’s it in a nutshell, however adopting this strategy will take time and practice and some unlearning of old ingrained habits, and the following examples may help. Note: on the highway at constant speed, improving efficiency is a lot simpler, basically reducing your cruising speed down to about the 50 mph range (depending on vehicle) improves efficiency more than almost anything else. You basically just have to balance the value of a few minutes saved on a typical trip vs extra fuel consumed (and perhaps safety) for travelling at higher speeds. Efficiency exponentially worsens in proportion to speed above 50 mph. Below 45-55 mph most engines (except some smaller engines) are not in their efficient RPM range.
It may come down to psychology that people feel they are “driving normally” when they balance the g-forces for accelerating and braking (which often will not get you to you destination any faster). But just because most people do it doesn’t make it “right”. For efficient driving, one can still be rewarded with relatively high g-forces for accelerating and if coupled with relatively low (or optimally zero) g-forces for braking or coasting, one can still have an enjoyable drive that’s efficient. You can have your cake (good fuel efficiency) and eat it too (accelerate quickly). If braking quickly (and following the crowd) means a lot to you, you can enjoy more frequent visits to the gas station and the brake shop along with significant more cost over the life of the vehicle.
EXAMPLES:
On an on ramp entering an expressway/highway that is free flowing at the speed limit, one should accelerate at a rate that keeps the engine RPM close to the RPM that produces maximum torque (or minimum BSFC if you have the RPM figure for this) until you reach the desired speed. This RPM figure is usually found quite easily for most cars using your favorite search engine.
If you need to go around a corner in the distance, take your foot off the accelerator at the distance that will allow the vehicle to gradually coast down to such a speed that will allow you to turn the corner safely without using the brakes.
If a light is turning red or already red in the distance, take your foot off the accelerator at the distance (before the light) that will allow the car to be moving relatively slowly (<10 mph) by the time it gets to the light. Every halving of speed from coasting, quadruples the amount of fuel saved relative to braking at the last second So if cruising (driving) speed is 30 mph 4 times more braking energy is wasted to slow to 0 mph versus slowing from 15 mph. From 60 mph it’s 16 times the wasted energy versus 15 mph. All that energy came from the fuel that you used to accelerate up to that speed. Also, there is a higher probability that you will still be moving when the light turns green again further reducing the fuel needed to get back up to speed.
CONCLUSION:
One of the worst ways to drive inefficiently is to accelerate very slowly and brake at the very last second. This is the driver who says “Manufacturer X is lying about their MPG, I drive the car very gently and still get bad MPG”. The only thing worse than this is accelerating at near maximum acceleration all the time and braking at the very last second. At least this driver will usually not blame the vehicle manufacture and know where the reason lies. (It may be their way of getting rid of the frustrations of the day and feel its worth the the price). I feel more sorry for the person who accelerates slowly and brakes quickly, as they don’t have “the fun” and pay at the gas pump and the brake shop.
briank101 