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
Affects of Speed On Powertrain Efficiency
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: 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%.
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. Overall powertrain fuel efficiency (BSFC: fuel burned normalized by power produced) is dictated by three mechanisms:
- Volumetric efficiency: efficiency of inducting fresh charge and exhausting burned charge.
- Thermal efficiency: efficiency of converting chemical energy of the charge inducted into thermal energy (combustion) phased optimally with respect to piston position.
- 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]
Mechanical Efficiency
Modern transmissions are amazing! Not many years ago, auto manufacturers had to select gearing that would provide reserve torque under vehicle cruise conditions to avoid frequent downshifts (gradeability). However, modern transmissions and the associated control systems provide near-seamless shifting, thus allowing aggressive calibrations that keep engine speed extremely low under cruise conditions. Obviously, the faster the engine turns, the greater the parasitic losses, so adapting aggressive strategies to maintain low engine speed, and the associated minimal throttling, enables excellent mechanical efficiency. As vehicle speeds increase, and in turn the engine load required to move the vehicle increases, engine power at extremely low RPMs becomes insufficient. Thus, engine speed and the associated mechanical losses must increase. [Editor’s note: Andy says high engine speed is associated with minimal throttling because, to drive at a given speed in certain conditions requires a given amount of power. So, let’s say you want to drive 55 mph on a certain road — that might require 25 HP. Power is a function of RPM and torque, the latter of which is referred to as “load” and corresponds to your throttle opening. So if you want to go a constant speed, you can do that at a low RPM and high load (like if you drove in a really high gear, which would minimize throttling) or a high RPM and low load (where you’d see more throttling losses, like if you were in a low gear). In either case, the product of the RPM and torque output will be the same. -DT] The correlation between minimal achievable engine speed as a function of vehicle speed is again dictated by vehicle drag and the engine torque curve. As required load increases at a low engine speed, spark retard may become necessary to avoid abnormal combustion, which results in a thermal efficiency penalty. The combination of these three efficiencies determine overall powertrain efficiency and, hence, fuel consumption, as a function of vehicle speed. Drag increases as a function of velocity squared, so that alone suggests that lower vehicle speeds are best. However, if the vehicle speed is too low, engine efficiency is sacrificed by throttling requirements (assuming a spark-ignited engine with a 3-way catalyst). Thus, depending on the vehicle/powertrain combination, best fuel mpg will occur at a constant speed of around 20 to 50 mph. Which, of course, is miserably slow, especially on a highway. I don’t imagine anyone will actually drive 50 mph on a long trip, but maybe you could drive 65 instead of 80 and get some real improvement. Also, if your gas light just came on and you don’t see a gas station in sight, the smart thing to do would be to drop to 50 at that point, at least, since 50 mph is still way better than walking with a gas can to the next exit. Minor nit pick: “Affects of Speed On Powertrain Efficiency” Should be Effects
- Coast down the hill getting 99+mpg and basically using no fuel, then power up the next hill, repeat OR
- Accelerate gentely down the hill to gain a lot of “free” speed, so you have more momentum to carry you up the next incline? Please tackle this. But, it depends on the speed and the vehicle. If, for example, carrying speed into the uphill avoids downshifting or keeps it in cylinder deactivation mode you could still be better off. I tend to mostly coast downhill, but every now and then accelerate and carry momentum. Especially if I’m losing a little speed on the downhill – better to get a bit more out if it if I’m leaving deceleration fuel cutoff anyways. The point about older transmissions having fewer gears is precisely why you got better fuel mileage going faster. In a 2002 Dodge Dakota with the Magnum 5.3 you shift from fourth to fifth at ideally 4600RPM at 50MPH. Fifth gear goes all the way up to 116MPH. Running at 60MPH you’re pressing the throttle harder and using more gas going up hills on the interstate than if you were doing around 75MPH. This is because for one thing your coasting speed going from down to uphill would decrease less with the momentum of going faster, meaning your engine works less to keep you around the same speed. The other reason is because, as was stated in this very article, mechanical efficiency isn’t a perfect 100% total vacuum never loss situation. Getting stuck behind another car that you could’ve passed uses more fuel when you have to wait and then speed up to get around them. Rough road surfaces have more friction, requiring more throttle to maintain speed, using more fuel. ” … hell, we knew this in the 1970s when the 55 mph speed limit law was passed … ” That was based on debated and partially debunked science regarding outdated engine efficiency charts and flat highway travel. The plains of Iowa are not the entire world and even by the ’70s the public didn’t drive flathead sixes with three speed manuals tied to them as we had in the 1940s. Y’all can have your fun. I’ll get there when I get there, enjoying higher MPGs. At the same speed, the stock 1.0L engine would be knocking at the door of 100mpg under steady cruise conditions, mind you.
