Powertrain Showdown, Part I: Conventional Powertrains
Meet “The Neverending Article.” Sure, it seems like a pretty straightforward proposition: Compare and contrast the major motivators out there today. No problem, right? And it probably wouldn’t have been 30 years ago, when powertrain options were limited to your choice of either gas or diesel. However, the 21st century isn’t like the 20th. Here in the future, the minute you think you’re done writing about one kind of powertrain, another one comes along, and you’re right back to re-drafting your fifth introductory paragraph.
Keeping up is a Sisyphean task indeed; every time you think you’ve got that boulder to the top of the hill, a hydraulically driven spring accumulator shoves it right back down to the bottom. For as many times as we’ve run over the different methods of motivating an automobile, I’m pretty sure they’re halved compared to the number of times I’ve been run over by that boulder. But, that’s exactly what makes today so exciting.
In this article, I’m going to go over the pros and cons of the major powertrains on offer today: internal combustion (gas and diesel), hybrid and electric. Of course, these days, those are far from the only options on the table. With internal combustion engines having hit a likely impassable developmental wall (see “Seven Unexpected Reasons Why Electric Cars Are The Future”), we’re right back to the Wild West days of blue-sky thinking and anything-goes engineering. For a few of those, check out Part II of this article, “The Ultimate Powertrain Showdown – Weird and Wonderful.” That was originally the second half of this one, but it seems certain editors aren’t too fond of 4,700-word articles.
Still…what are you gonna do? Like the future itself, this is the article that never really ends. Like Sisyphus pushing his boulder up the hill, or writers on their fifth page-one rewrites, it seems we’ve all got a load to push if we’re going to undo the mistakes of the past. But it’s a welcome burden. When you’re finished with this one, make sure to check out Part II…to watch that rock roll downhill again.
Continue Reading for the whole story
This is as good a place to start as any. To understand the pros and cons of both gas and diesel engines, you have to go back to why they rose to prominence in the first place. Back in the day, 1880 or so, power was hard to make and energy was hard to store in a usable form. Steam engines had been around forever, but they were heavy, complicated and cantankerous. Electrics worked great, but batteries of the time were so heavy and energy-poor that they just weren’t practical.
The quicker you can burn fuel, the more horsepower you'll make, and gas engines can burn a LOT of fuel.
The first liquid-fueled internal combustion engines ran on heating oil, an expensive commodity as likely to come from the heads of sperm whales as a hole in the ground. Since fueling America on marine mammals was never a long-term plan, a cheaper fuel had to be found. Fortunately, the kerosene-refining industry made loads of this nasty, volatile, energy-rich waste product called "gasoline." It took a while to harness its power, but gasoline was at least very, very cheap.
The first gas engines were essentially diesels modified to use gas, which burned a lot faster and required a spark plug to perfectly time ignition. From the outset, these engines existed for one purpose: to make loads of horsepower in a lightweight package, burning lots of a very cheap and energy-dense fuel. All other considerations were secondary.
- Horsepower: The quicker you can burn fuel, the more horsepower you’ll make, and gas engines can burn a lot of fuel. Because the fuel itself burns very quickly, you can have many combustion events per second; that makes for very high rpm, and horsepower to go with it. As of right now, little else on Earth makes usable high-rpm horsepower like a spark-ignition engine does. They burn a lot of different fuels nowadays, but that much remains constant.
- Weight and Packaging: You might think this is an antiquated idea, given the size and complexity of today’s car engines. But look on a smaller scale, at something like the incredible Rotax. These engines (often used in snowmobiles) make hundreds of horsepower in a package little bigger or heavier than your average aardvark. The same could be said of any number of motorcycle or formula racing engines, which make up for their tiny displacements with 10,000-plus rpm redlines.
- Efficiency, Especially at Part-Throttle: Gas engines were never designed to be efficient, and they still aren’t. For most designs, that’s especially true at part-throttle. Gas engines have a throttle plate, which restricts airflow at idle and part-throttle. The intake manifold vacuum behind it represents something called "pumping losses," which account for a stunning loss of fuel economy at part-throttle. It’s not something that’s easy to get around, and most gas engines don’t. Unless you drive everywhere at wide-open throttle, that’s kind of a problem in terms of efficiency.
- They Have to Idle: Yes, we do have gas vehicles today with start/stop setups that eliminate idling. But those systems rely on a powerful electric motor to get the vehicle moving initially. Basically, it’s just a glorified starter. But still, that makes these start/stop vehicles technically hybrids.
- Torque: It’s funny, we tend to think of gas engines as having a lot of torque. But actually, sheer force is one of the gas engine’s weakest points; especially when you compare it to diesel, electric, steam or pretty much anything else.
Gas engines are great at making a lot of horsepower, but not much good at anything else. They’re still the powerplant of choice for making absolute power, even in hybrids like the Koenigsegg Regera.
But gas engines are still a fuel-hungry compromise, at best.
Compression-ignition engines have one major advantage going in: the fuel they burn is more energy-dense than gasoline. For that reason alone, diesels will always return about 13 percent better fuel economy than any comparable gas engine. That in itself isn’t enough to save money, though, since diesel is usually at least 15 percent more expensive than 87 octane. But they have other advantages that easily offset that. Ironically though, today’s best diesels are getting more and more like gas engines to make up for their comparative lack of horsepower.
Geared correctly, diesels can offer massive corner exit speeds.
- They’re more efficient at idle and part throttle: Diesels don’t have a throttle plate. No throttle plate means no intake vacuum, which means little to no pumping losses. That makes diesels far more efficient than gas engines at anything other than full throttle — in other words, where you spend 99 percent of your time. Probably.
- Low-rpm torque: Diesels specialize in low-rpm torque, at least compared to gas engines. That means not only lower gearing and lower rpm, but usually a lower idle. Not by much, granted — a diesel will usually idle at about 500 rpm, compared to a gas engine’s 750 to 800 rpm. Still, proportionately, that’s a 50 percent reduction in idle speed, and a concordant drop in fuel consumption. On the fun side, diesels offer a massive broadsword of a torque band. Geared correctly, diesels can offer massive corner exit speeds with minimal effort from the driver.
- They can run on vegetable oil: If you’re into brewing your own fuel, diesels have it all over other designs. Diesel fuel is, to put it plainly, kind of crappy. These engines are designed to run on anything short of liquid tar, and they’d probably run on that for a while. From collecting vegetable oil to squeezing algae, the fact that diesels can use fairly unrefined fuel makes them a lot more versatile in terms of the fuel source.
- Horsepower: Diesel fuel burns slowly, which means engines using it are rpm-limited. Since rpm is horsepower, diesels have to make up for it with turbocharging and huge torque. That’s not necessarily a bad thing, but it is a weakness compared to gas engines.
- Weight and Cost: This isn’t as true as it used to be, but diesels are usually heavier and more expensive than gas engines. Then again, modern cars are also heavy and expensive. So, maybe it’s all relative.
- Emissions: Diesels today are a lot cleaner than the oil-burners of yore, but there’s a problem inherent to them. It’s not that diesels produce a lot of emissions, it’s that they produce much higher levels of nasty nitrogen oxides. NOx emissions form when nitrogen and oxygen come together at kinds of high temperatures and pressures you’ll find in a diesel combustion chamber. It’s almost inevitable, just by the nature of compression ignition.
Compression ignition engines are more efficient than gas engines everywhere but wide open throttle, and more driveable depending on how they’re geared. You might have to give up some high-rpm horsepower, but that’s nothing cranking up the boost won’t fix.
Electrics came very close to winning the bid for the automotive market back in the day, and for good reason. The motors themselves have almost no downsides. For the sake of brevity, read “Seven Reasons Why Electric Cars Are the Future.” But the long and short of it is this:
- Instant off-idle torque and no on-site emissions.
- Torque vectoring for ultimate handling.
- Quiet and simple operation.
- They can run on anything a power station does, including sunlight.
- One moving part and no transmission means less to break and longer-lived automobiles.
- Good ones are still absurdly expensive for a vehicle with effectively no powertrain.
- Still waiting for battery technology to make them a fully viable alternative.
- Still waiting for the SAE to standardize battery packs so they’re fully interchangeable at service stations, thus eliminating recharge times altogether.
- Still waiting for Luddites to acknowledge we need them before brown polar bears start showing up on South Beach.
The powertrain of the future, waiting for the future to catch up.
There are all different kinds of hybrids.
As of right now, hybrids seem to offer the best compromise between gas and electric, or diesel and electric as the case may be. There are all different kinds of hybrids, with different degrees of electrification and reliance on the electric motor or gas engine. In this case, we’ll break down the pros and cons of the two main hybrid types.
Series Hybrid — An engine charging the batteries (e.g. Chevy Volt)
- The most fool-proof design, offering consistently high gas mileage because the computer is in full control of when the engine’s running and how much fuel gets burned.
- Offers the most brake regen capacity, since it’s driven entirely on powerful electric "traction" motors. The more powerful the motor, the more brake energy it can recapture.
- Usually has more battery capacity, allowing it to operate more on electric only if used as a plug-in hybrid.
- Ultimately not as efficient as a parallel hybrid. Second law of thermodynamics (entropy) says you lose energy every time you convert it. The more energy you convert to electricity, the more you waste in that conversion. Thermodynamically speaking, it’s more efficient to use the gas or diesel engine to drive the wheels directly than to run a generator to charge the batteries.
Parallel Hybrid — Engine both charges the battery and drives the wheels (e.g. Porsche 918)
- Theoretically more efficient than a series hybrid.
- Usually offers more combined horsepower than a series hybrid, because the gas engine helps out on acceleration. That may change in the near future, though.
- Can’t recapture as much regenerative braking energy, owing to less-powerful electric motors.
- Can absolutely guzzle fuel, depending on how hard you drive it. As Top Gear found out, it is possible to get 17 mpg out of a Prius if you thrash it hard enough.
Better than gas or diesel, without the range anxiety (or energy efficiency) of pure electrics. If you can keep your foot out of it, parallels are better in terms of efficiency in most cases. However, less-powerful but more-predictable series hybrids are more likely to return the best fuel economy on average.
Conclusion, Part I
Like a lot of things in life it’s not really a matter of what’s best — it’s a matter of what’s best for a particular set of circumstances. There’s no intelligent design in the evolution of powerplants…what works for one application, at one time, under one set of circumstances, won’t necessarily survive in another.
Gas engines make a lot of horsepower at wide-open throttle while wasting a lot of fuel; diesels make more torque at lower rpm while wasting a bit less fuel. Electrics waste very little energy, and can offer the best of all possible worlds. But until battery and clean power generation technology catch up, we’re stuck using gas or diesel engines to extend their range. This is a transitional period for the automobile, kind of like the turn of last century; it’s not hard to find plenty of heavy parallels between 2015 and 1915. Especially considering the fact that both hybrids and electrics have made a comeback from that time…and they’re not alone.
Stay tuned for Part II, when the boulder rolls back down.