Let’s talk about motors and engines. Arguably, they’re kind of important to the driveability of your car or truck. Those of us who have ever had to try pushing a vehicle down the road can attest to how truly wonderful it is to have a different motive source of power than your own two legs. But, an electric motor is not a gasoline engine and it’s not a diesel engine either. They all three are different beasts with different advantages and disadvantages.
The obvious thing people want to know is “how many horsies does she got?” As luck would have it, you can answer this question for any of the three. A Bolt has a 200HP motor. Tesla vehicles have a range but 350HP is one option. I own a 1985 Mercedes that can make 230HP. Which one is faster? Obviously the Tesla leaves the others in the dust. However, what might be more interesting would be to consider the performance of the Bolt vs my Mercedes. Which one wins? Well, the fight isn’t entirely fair as the Mercedes can do 135MPH while the Bolt is governed to 93MPH. So, after 93MPH there is a clear winner (probably the police!) But, what about the 0-60MPH times? A Bolt can do 0-60MPH in 6.5 seconds. This is glacially slow compared to a Model S Plaid but, I assure you, feels quite zippy when you do it in person on a real road. What about my more powerful V8 Mercedes? That goes 0-60 in 8.9 seconds. Wait, the Mercedes has a 30HP advantage and an engine the size of a small bus. Why is it so much slower? Could it be that focusing on horsepower is a bad way to compare different motors / engines? Yes, it could be and is a poor comparison.
You see, horsepower is one of those things that sounds like it’s just some formless unit engineers made up to annoy people. However, in fact it has more lurking in the shadows. A horsepower can be defined a few ways but the mechanical definition is 746 watts if you mean imperial mechanical horsepower (and this is ‘Merica damn it!) But, more importantly for this discussion, horsepower is (torque * RPM). This is glossing over the fact that you divide this by a set factor. But, this factor is a constant and so doesn’t affect the discussion. So, double the torque and you double your HP. Double your speed and you double your HP. This means it’s a bit meaningless to say “it’s got 400HP!” No, it doesn’t, not all the time. In either an electric motor or an internal combustion engine there is a maximum amount of torque you can generate at any given moment. So, your output HP is always going to be relative to the torque you can provide multiplied by rotational speed of the motor. Both combustion engines and electric motors tend to use gearing for an important reason – it’s easier to spin faster in a smaller motor to get your HP instead of directly generating more raw torque. This is why a Tesla motor can spin up to 16k RPM. This is much easier than trying to build a motor that can spin 1.6k RPM directly to the wheels with the same power. So, the Tesla Model S uses a quite high ratio of around 9.7:1. This keeps the motor small and light (comparatively) while still allowing for high peak power.
But, there are two elephants in the room. It’s quite crowded in here now! The first elephant is called “gas engines don’t have such great torque at low speed”
This is ultimately why the kid in the beat up rust box next to you is revving his engine at the light. He needs the RPM to get any sort of power at all. Meanwhile, an electric motor has all of the torque it will ever have right up front. At 5 RPM it basically has just as much torque as it will have at 1000RPM. This goes a long way to explain why a 200HP Bolt motor can make a 230HP V8 look like a pile of junk in a race from 0-60MPH. The V8 has two disadvantages. First, it doesn’t generate its full power until it has revved up a bit and secondly, it has to shift. Ultimately shifting is necessary in cars with combustion engines because they have a somewhat limited window where they generate a lot of power. Notice how the torque of the gas engine drops even faster than the torque of the electric motor. Engineering Explained covers this quite well (though the S2000 actually keeps its torque very well!)
So, what’s the second elephant in the room? Here is a Chevy Bolt Motor:
Note that this has the gearbox on the end. It weighs around 167 Lbs / 76kg which is isn’t light but lighter than me. This is what a 200HP electric vehicle motor looks like. Meanwhile, this is what a 200HP industrial motor looks like:
How much would you estimate that little guy weighs? 500 lbs? Nope! 1000lbs?! No. A ton?!! NO. It weighs 3004 pounds. Yes, 1.5 tons. So, why is the industrial motor 17 times the weight of the EV motor of the same rated power? Well, two reasons. 1: Nobody cares what an industrial motor weighs. If your forklift can lift it then it’s light enough. 2: That industrial motor can be run at 200HP all day, every day, all year, every year. That EV motor? 200HP is quite an imaginative number. Can it do 200HP? Well, for a while. But, advertising it as 200HP is like saying I could flip a 200 pound tire. Yes, I probably could… once. Maybe twice. Certainly not all day. And, so it is with motors in electric vehicles. Yes, they can do their rated power for a while but the more you push them, the more they heat up. This also is something to keep in mind while comparing electric to gasoline. A 200HP gasoline motor can more or less do 200HP until you run out of fuel. As such, when doing EV conversions, it bears mentioning that electric motors tend to have two values of importance when it comes to peak power. There’s the “geewhiz, if you floor it you can get this much power!” figure and then the “if you drive 100 miles down the road you can expect to maintain this level of power” figure. This figure is much more conservative. Take, for instance, the UQM PowerPhase 100 that we sell. As you might guess, it is 100kW peak power. The continuous rating is instead 60kW. This is not bad either but it’s not 100kW. One thing EV motors do have going for them is liquid cooling. With liquid cooling you can make motors far smaller than they’d otherwise be. Apparently 17x smaller! Even Tesla motors cannot beat physics. Many people have found that racing a Tesla on the track is fun right up until the motor goes into thermal limit. Ultimately a Tesla Plaid may generate more than 1000HP for a few runs but you are not going to be towing cross country a semi-trailer loaded with 40,000 pounds of hamburger.
1 thought on “The little engine that could”
Gasoline engines also have a continuous power rating based upon the amount of cooling that you can deliver to them. It’s simply that methods have been developed to cool them adequately. Most production cars running these ICEs have enough cooling to always be sufficient for the engine to run at peak power.
With electric motors, it’s not possible to cool them to the same degree with the same methods. This is why efficiency is important, to reduce the amount of heat generated. This allows the motor to increase its continuous power rating.
I have a custom-built electric velomobile with a Leafbike 1500W in-wheel hub motor. Its manufacturer rated the motor slightly conservatively to 1500W at 48V, but in reality, it is closer to 2 kW continuous. It can also handle 7-10 kW peak as ebike builders have done so without destroying anything but the occasional Hall sensors, but only for a small number of seconds at a time. The motor and its components can’t shed heat fast enough to avoid damage after about 2kW, the most critical limiting factor being the size of the phase wires. I’ve added ferrofluid and a hubsink to it to allow the heat generated in the motor to conduct itself to a series of fans that attach to the motor case and rotate with the wheel. If I were to run a 72V pack, it may become a 2500W-3kW continuous motor due to the added cooling, while still only being able to about handle 10 kW peak, for roughly the same time duration if not a few seconds longer as it would with no cooling. Someone on Endless Sphere deliberately melted the windings in one, and found it took 20 kW to do so, at stall, for a period of time measurable in minutes(but long before it melts, components critical to its functionality in a vehicle will start failing). The motor peaks at 90.7% efficiency, and typically gets ~85% efficiency during most of its operation. This motor weighs 15.7 lbs.
Rather impressive for the weight considering it is a PMDC hub motor, right? Not really. This motor has been on the market for a decade. Much better is possible. The more efficiency a motor has, the closer its peak power and continuous power approach each other, without any added cooling. And the state of ebike hub motors on the market today is dismal. The Leafbike is about the best choice readily available for a direct-drive micro-EV application. It’s a low bar to beat, but no manufacturer is doing it.
Consider what is possible. In 2013, a company called AMZ technologies made a prototype motor that could do more than 98% peak efficiency. The 7 lb motor was capable of 46 horsepower peak, and 28 horsepower continuous, without any added cooling, and about 280 lb-ft of peak torque.
That’s no joke. Too bad we can’t buy them.
Think of the automotive design possibilities something like that would open up. After all, it’s not about how much power you have, it’s what you do with it. That is why the Bolt is so much faster than the Mercedes, in spite of having less power available.
This motor being available would open up the possibility of an enclosed 1-seater microcar with driving dynamics inspired by a racing shifterkart or Formula SAE car, a focus on efficiency inspired by a velomobile, and EV motors light enough to keep the vehicle’s mass in the sub-motorcycle range while having AWD with vector control and slip detection. This design consideration opens up the possibility of a platform that only needs about 20 Wh/mile to do 70 mph on the highway, allowing the battery pack size to be kept small for an acceptable range under normal driving conditions. This is done by extreme aerodynamic drag reduction, by taking on the footprint and aerodynamics of a velomobile.
Such a vehicle, built of a combination or motorcycle and ebike parts, could make excellent use of a 5 kWh pack of Lonestar batteries. This is a pack that would only weigh about 70 lbs. It could also make many hundreds of peak horsepower until dead, well more than we’d need for this proposal.
4 of those motors would weigh 28 lbs. Controllers that can run each motor have been made that are 1.5 lbs each.
You see where I’m going with this yet? Even with a roll cage designed to kep the occupant from getting crushed by multi-ton vehicles on the interstate, we’re talking about a sub 200 lb “car”, that makes 1 peak horsepower per pound of vehicle. With AWD, slip detection, and vector control. What kind of performance would that have? Math suggests 0-60 mph in 1.9 seconds(limited by tire compound) and 0-120 mph in 4.5 seconds is possible. Due to low mass and a focus on drag reduction instead of downforce, it would be at the limits of design possibility to have stability much above 120 mph, so somewhere around there would be the likely ideal stopping point for acceleration, although that would take a lot of aerodynamic design study to determine just how fast you could make such a thing go safely. But I digress, as that would still be a stupid level of performance when you consider the savings of the cost of building such a vehicle compared to how much it costs one to buy cars that offer that sort of performance. Bonus: if the body is designed to be twice as drag-inducing as a Milan SL velomobile, it would cruise 120 mph on only 6 horsepower, WELL below the continuous ratings of the motor. 200+ mph continuous may be possible with all 4 of them motors, if(very big doubtful if) basic stability can be achieved while keeping the drag that low.
In mass production, the EV components needed would be stupid cheap compared to what it costs to convert a normal sized car. You’d be looking at a few thousand dollars in controllers at low-purchase-volume retail cost, which would go down with mass orders. A 5 kWh battery isn’t much money either these days, in the low 4-figures for even the most powerful racing batteries. The motors I mentioned above that we’d need are currently unobtanium, but if volume production commenced it isn’t inconceivable that they could be made as cheaply as the Leafbike motor I use, which set me back a grand total of $350 including shipping it across the Pacific Ocean. We’d need 4 of them. So parts cost is MUCH less than you’d need for a full sized car, or even a motorcycle(thanks to battery requirements being low for appreciable range). This thing might be producible for the cost of an inexpensive gasoline motorcycle, if you can get the production volume high enough. If you can’t? That kind of performance would still justify a 6-figure price tag to hand build, on its own merit.
So, a vehicle that weighs under 200 lbs, does 0-120 mph in 4.5 seconds, and tops out at 120 mph? AND costs fractions of a penny per mile to operate on the interstate? Where the most expensive parts that can fail will be replicable for only a few hundred dollars? Where every 120V outlet is the equivalent of a ChaDeMo on a normal sized car and every 240V outlet is equivalent to a Tesla Supercharger in terms of miles of range recovered per minute?
Today’s technology would allow for such a thing.
The vehicle I’ve built is much more modest. It tops out at 45-50 mph depending on state of charge using only 700W plus hard full-effort pedaling, is pedalable to 35 mph in a sprint with the motor disabled, weighs 91 lbs, and when using the motor it can peel out and do donuts with ease even though it’s only using a 48V battery and I have the controller limiting the peak power to 3 kW. Its 0-20 mph acceleration is probably comparable to Jack’s 50 Spyder that I drove in 2013, but due to low power, the acceleration rapidly tapers off after that. It cruises 30-35 mph using only 8 Wh/mile with light pedaling. I get a 150-200 mile range @ 30-35 mph with a 1.5 kWh battery pack. If I increase that speed to 40 mph, range would still be 100 miles, but the vehicle is not yet built to reliably/safely handle much over 30 mph cruising. It’s not nearly as impressive as what I proposed above, especially regarding performance, safety, and aerodynamics, but a basic proof of concept demonstrating the extreme efficiency that is possible is certainly there. With that sort of efficiency, performance will follow with it, because you need less horsepower to go fast.
If you’re in Cape Girardeau, perhaps I’ll pay a visit with it some day from St. Louis.
One day, I’d like to build something much more badass.
Little engines sure as hell can.