It doesn’t matter how long you’re into cars, there’s always something new to learn. Recently, I learned why some cars seem to have what appears to be worthless dead weight built in. It turns out that that sometimes, there’s good reason to add weight to a vehicle. Pretty often, it’s to make up for problems that an internal combustion engine brings to the table.
I won’t go through the whole story (you can find that at HACKADAY), but most of the time it comes down to vibrations. Most combustion engines have weight that moves back and forth rapidly. Great care has to be taken to balance the weight of pistons, connecting rods, crankshafts, and valvetrain parts that move. If these are even just a little out of whack, the engine will run poorly and slowly destroy itself. The designs of some engines (most often the common inline 4-cylinder) is naturally unbalanced, and can require extra moving parts to reduce vibrations.
No matter how hard they try, some engines are going to naturally vibrate more than others. Plus, road vibrations and other parts of the drivetrain can make their own vibrations. So, to make up for that, manufacturers have to find other ways to reduce the vibrations the car’s passengers feel. In some cases, adding some dead weight disrupts the harmonics of a vibration, seriously deadening it. In many older vehicles, the dead weight got added after the car was designed. Today, computer aided design makes it easier to design that extra weight into existing parts, effectively hiding the dead weight.
These are both small potatoes compared to the active weights some vehicles are starting to have. Like the tuned mass dampers found in some large buildings, some cars have small weights that are electronically moved in such a way as to cancel out engine or road vibrations. Or, the truly massive weights that come in some trucks to keep them from flipping over.
Smoothness Comes At A Cost
Adding extra weight isn’t free. Sure, the vehicle can feel a lot smoother and more luxurious with well-placed extra weight, kind of like humans. But, carrying extra weight around means lower fuel economy, reduced handling, and extra cost (both at the time of purchase and with fuel and maintenance).
This isn’t the first time comfort and “refinement” have come before efficiency. Another great example of this is the average “slushbox” automatic transmission. Ideally, an automatic transmission should shift fast and hard, minimizing the wasted energy, excess heat, and premature wear that slipping causes. But, many people think there’s something wrong or uncomfortable if their car gives a rapid, jarring shift that you can hear and feel. So, to make things smoother, most manufacturers decided decades ago to let the transmission slip a bit for comfort, but at the cost of in-town fuel economy and overall durability.
Other manufacturers decided to try to get rid of shifting altogether by putting in CVT transmissions, but as Nissan proved, that was a terrible idea that led to far less durability.
Other things, like soundproofing and vibration-deadening material, crash protection, and comfort/convenience features all make this same trade-off. You get something for the extra weight, but you still have to carry around all that extra weight.
EVs (& Some Combustion Engines) Mostly Avoid This
While a combustion engine can have hundreds of moving parts, an electric motor’s whole drivetrain from wires to wheels only has a handful. The rotor in the motor (the part in the middle that moves) only rotates in one direction while moving instead of moving back and forth rapidly. This means that you don’t need all of the extra weight, sound-deadening, vibration-deadening, and other measures to increase smoothness. You get a smooth motor right from the get-go.
And the transmission? It’s usually just a single-speed gear reducer and differential, with no actual shifting taking place. Even when shifting does happen, there’s only 2 or 3 speeds, with industry estimates for the future maxing out at around 4 speeds. Compared to the 6, 8, and even 10 speeds gas-powered vehicles are coming with now (there are patents for 11-speed automatics, now), there’s not only a lot less to go wrong, but no need for stupid measures to increase smoothness.
It wouldn’t be honest to not mention that there are alternative combustion engine designs that are inherently smooth like an electric motor, but there are good reasons we don’t see them in cars that much.
Turbine engines are a great example, and if your EV isn’t powered by renewables, it probably gets some of its electricity from the massive turbines at a power plant. But, these are expensive and not well-suited to directly powering a car with its mechanical energy. The other problem is noise. Almost nobody wants a little jet engine acting as a range extender, but for a few enthusiasts, it’s fun.
Speaking of RX-7s, there’s also the Wankel rotary engine. You can read all about them in my other article here, but in short, they’re combustion engines with only a few moving parts. They’re quite smooth, they’re compact, and easy to work on, but don’t tend to be very efficient or durable at the variety of speeds a car needs to go, but if they can move at a steady RPM, they do a lot better. So Mazda plans to use them in some range extended electric vehicles in the coming years.
EVs Do Tend To Weigh In Heavy
While EVs are the smoothest reasonable option in town, they’ve got their own weight problems, but they aren’t caused by dead weight. Battery technology has come a long way, but energy densities are still far lower than that of gasoline or diesel fuel. This means that to accomplish reasonable range on battery power alone, you need a lot of battery. This drives the weight of EVs higher than gas cars on average. But this is something that continues to improve, and isn’t going to be a problem forever.
Despite this, they’re still the superior choice for most applications. Even though they’re heavier, they’re still more efficient and environmentally friendly because they don’t turn 60-80% of their fuel into waste heat. Plus, the weight can be placed where it’s best for vehicle handling and safety instead of having to strategically place it in the vehicle for vibration and noise reduction.
Extra weight that serves a purpose (batteries) is definitely superior to extra weight that’s just there to make up for the shortcomings of old technologies.
Featured image: The Active Tuned Mass Module that comes on 2019+ Dodge Ram trucks. It moves its weight around to cancel out vibrations from the engine and the road. Image provided by Stellantis NA.
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