Here’s Why You Should Slow Down To Save Gas According To An Aerodynamicist And Powertrain Engineer

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As I’m sure you’re already aware, gas in America is stupid expensive right now. Well, stupid expensive by American standards, which are, admittedly, not the same as the rest of the world, where I believe gasoline is sold by the hip flask in exchange for human kidneys. Currently in America the average price for a gallon of gas is $4.40, and that adds up fast here in the land of vast distances and thirsty cars. There are ways to cut down on the gas you use, but they’re generally no fun. So, to help, let’s sweeten that medicine with the sugar of science, straight from some experts.

There’s actually two types of experts we’ve reached out to in hopes of helping us understand how to save gas, an aerodynamicist and a powertrain engineer; both areas play into how many miles your car can go on each precious gallon. Let’s start with our engineer named Austin Wright, who majored in aerospace engineering and who works for a major automaker as a calibrator.

Then, we’ll go into the powertrain side of the equation, which will be laid out for us by ECR Engines Technical director Andy Rudolph, who helps develop NASCAR engines.

I’m sure you’ve heard that driving slower will use less gas. This feels intuitively correct, and plenty of studies have backed up claims like how driving at 60 mph uses 14% less fuel than driving at 70 mph. If you have a car that’s rated to get 25 mpg highway, that’s the difference between getting 22.5 mpg and 25 mpg, which, if you’re going on a, say, 500 mile road trip would be a bit over two gallons saved, which is between $8 and $10 depending on where you’re getting gas.

That’s something! But, nobody likes driving slower because, well, it’s slower. And when you’re on the highway, it hardly seems like you’re working your car that much harder at 70 mph compared to 60. But there’s a lot more going on, both regarding aero and your engine, which is why I’m going to pass this off to Austin now.

Aero Considerations

The primary reason efficiency tanks with speed is due to drag.

Buckle up, lets go

Drag increases exponentially with velocity, as I’ll show below. Let’s look at the drag equation:

Drag coefficient is a fixed coefficient, determined through testing in a wind tunnel. Air density and frontal area won’t change either, so we can shove these terms into a black box I labeled ‘constants’.

Meanwhile, velocity can change, and it’s squared. In a nutshell – the force of drag depends on the constants we are boxing up, but it doesn’t change based on them.

Drag only changes based on velocity. And, by being squared, drag increases exponentially with velocity. 

I’ll quantify how drag increases with velocity. We’ll look at the difference in drag between 65 and 80 mph:

I converted mph to feet per second, which is the units this equation likes (or m/s if you prefer). As I demonstrated above, a 15 mph difference causes drag to increase over 50%!

So, how does increased drag relate to fuel economy? Your engine burns fuel, and through that chemical process, a transmission, etc, accelerates the vehicle. Think of that acceleration as a forward acting force (Force is mass times acceleration – Newton’s 2nd law).

As a driver, when you press the pedal, you feel that force accelerating you forward! Once moving, drag creates an opposing force, ‘pulling back’ on your vehicle. To accelerate, the force from your engine must exceed the drag!

To travel at a constant speed, your engine has to match the drag produced. Knowing this, a car’s top speed is defined by its power output and its drag characteristics. At top speed, the car isn’t accelerating; max power in the highest gear is equal to the drag produced.

A great example you could use to demonstrate the exponential nature of drag is the Bugatti Veyron.

With 987hp (1001PS), the Veyron hit an average top speed (on ) of 254.04 mph. The Veyron Super Sport came out with another 197hp, for a grand total of 1184hp (1200PS). The Veyron Super Sport hit an average top speed of 267.86 mph.

Due to the exponential increase of drag with velocity, the Veyron Super Sport’s 20% increase in power output only increased its top speed by 5.16%.

 

(This is a great, but not perfect, comparison. Both top speeds were recorded at VW’s Ehra-Lessien test circuit, and the cars are relatively similar. But weather conditions could affect the results slightly. Additionally, the Veyron Super Sport had aero changes to reduce its drag coefficient, like sleek NACA duct intakes replacing the previous, upright intakes.)

Notice that weight is absent from the drag equation. This is why the Veyron and Chiron can be so heavy! Weight certainly affects acceleration (inertia), but it has little effect on top speed (beyond rolling resistance with the tires). Additional weight actually makes the car more stable at speed.

That’s a lot to take in, but I think it’s good to see everything broken down. Essentially, you have to think of air as this invisible goop we all swim in as we live our lives, and as we move through that goop, we need to push it out of the way.
If your car was shaped like a needle, this wouldn’t be a big deal, but it isn’t; it’s closer to a smoothed-over box. So, the faster you shove that box through the goop, the more goop it has to go through, and what Austin’s math there showed was that going 80 mph through the air-goop makes that air goop impede you by 50% more than going 65 mph.
That’s a lot of air-goop drag.
Just because we can’t see air doesn’t mean it’s not there, of course, and going slower through the air simply takes less effort, and as a result, fuel for your car to burn.
Okay, that’s a great explainer of the aero side of the equation, but now it’s time to dig into the mechanics of everything. Let’s get Andy Randolph to explain how your car actually uses fuel, and what affects its thirst for it:

Affects of Speed On Powertrain Efficiency

First, I am assuming you are referring to a constant vehicle speed on a level road, in a spark-ignited engine running at stoichiometric air-fuel ratio.  Obviously, fuel economy worsens whenever load transients are introduced, particularly if they involve braking or stopping.

Overall powertrain fuel efficiency (BSFC: fuel burned normalized by power produced) is dictated by three mechanisms:

1) Volumetric efficiency: efficiency of inducting fresh charge and exhausting burned charge.

2) Thermal efficiency: efficiency of converting chemical energy of the charge inducted into thermal energy (combustion) phased optimally with respect to piston position.

3) Mechanical efficiency: losses due to rotating and reciprocating friction of the powertrain mechanical components plus power requirements of ancillaries (pumps, drives, electrical).

Let’s discuss each of these individually and relate how are impacted by vehicle speed.

Volumetric Efficiency

Volumetric efficiency is dictated by the flow efficiency of the intake and exhaust systems, and the amount of throttling employed.  Modern dual-cam-phasing systems provide an excellent means of load control that reduces throttling losses. The two primary mechanisms are charge dilution via internal EGR (exhaust gas re-ingested into the intake tract) and late intake valve closing (reducing the effective compression ratio).

When loads are extremely low (low vehicle speeds), it becomes impossible to avoid throttling. Thus, efficiency is lost if vehicle speed, and hence required engine load, is too low. If the load is too high, efficiency is good but fuel consumption increases to provide the necessary motive power. The ‘sweet spot’ for fuel mileage is typically around 20 to 50 mph, depending on drag characteristics of the vehicle and engine displacement.

[Editor’s note: Imagine a throttle plate that’s barely cracked open, and think about the sound you tend to hear as the air gets sucked through that small opening — there are lots of pumping losses or throttling losses associated with the restriction. That’s what Andy is talking about in the second paragraph in this section. EGR is inert gas that goes into your engine, and takes place of air. One of its key benefits is that it lets you open your throttle fully (to get fewer pumping losses) while still keeping loads (and thus vehicle speed) down. -DT]

Thermal Efficiency

Thermal efficiency is also enhanced by independent cam phasing when implemented properly.  For instance, a late-late valve timing strategy (delayed exhaust opening, delayed intake closing) provides pumping loss reduction by replacing throttling with late-intake-valve closing, while simultaneously increasing expansion ratio by delaying exhaust valve opening.

There is not an exhaust pumping penalty with this strategy at light engine loads because exhaust mass is low. In fact, delaying exhaust valve closing optimizes the exhaust event by balancing blowdown losses from opening the exhaust valve during the expansion stroke with pumping losses by not having the exhaust valve fully open early in the exhaust stroke. As with volumetric efficiency, this strategy for increased thermal efficiency is very effective at engine loads corresponding to vehicle cruise speeds between 20 and 50 mph.

[Editor’s note: If you didn’t 100% understand this, don’t worry, I’m not sure I do, either. But the point is that modern variable valve timing systems allow an engine to change when and how long valves open and close to maximize efficiency. Typical understandings of how vehicle speed/engine load affect efficiency (like the throttling losses we mentioned earlier) need to be reconsidered with this technology. Andy is saying that, even with this tech, thermal efficiency is maximized between 20 and 50 mph. -DT]