As hybrid cars become more common, are we going to see series hybrids with exotic engine designs?

Exotic engine designs : there’s a large number of them. Many of them run at high RPM or have limited lifespan or would be very difficult to connect to a transmission.

But what many of them have in common is that they are simpler, sometimes more efficient, and provide more power (but probably not torque) than a piston engine of the same size.

Rotary engines like this one that Liquid Piston has.

1 cylinder engines:

Jet Turbines :

Basically, the car would have one of these compact engines driving a generator (or in the case of the mono-piston, the generator is integrated into the engine walls). There would be far fewer parts to the engine, and it would be designed for a shorter lifespan, making it cheaper.

One serious problem is that it really ought to be able to consume 1 year or long gummy gasoline, because the idea is, you ship an electric car without half to 2/3 the pack size of the Bolt or Tesla. That gives you 100-150 miles of pure electric range, enough for most people’s reasonable commutes. But when necessary, the engine will start at about the 20 to 30% battery charge point.

It would ideally be sized that at peak output, it provides the average electrical power needed to keep the car moving at realistic highway speeds (70-80mph) at max design load (5 people in it + cargo). I’ve read that’s around 60 hp. (44 kw)

It seems like you ought to be able to build such an engine for less than adding the additional battery capacity…

What you’re describing is basically the BMW i3 with Range Extender. It has a 2 cylinder motorcycle engine to charge the battery when needed. It solves the ‘gummy gas’ problem by periodically starting up and running for a few minutes. The petrol tank is only 9 litres so it doesn’t have much time to go stale.

Decoupling instantaneous engine power requirements from instantaneous wheel power requirements does indeed give you a lot more options as to what sort of powerplant can be used.

One reason conventional gasoline engines suck is that they…suck. Literally. They spend most of their time with the throttle plate mostly closed, and part of their power output is devoted to sucking air past that throttle plate. Diesels do better in that regard.

Another reason conventional gasoline engines suck is that they have to use a rich enough mixture to sustain a flame front all the way across the combustion chamber. If it’s too lean, the flame front fizzles out and you get a partial burn, bad for efficiency and emissions. But since you can’t use too much excess air, you get high combustion temperatures which dumps a lot of heat to the cylinder walls, bad for efficiency. Once again, the diesel does better in both regards: it solves the combustion problem by compressing things to the point that they’re so damn hot, any fuel you squirt in there is going to burn, no matter how lean the overall mixture is. And because it’s so lean, peak combustion temperatures are low, and you lose less heat to the combustion chamber walls.

Where diesels suck is in emissions: with so much excess air they produce a lot of NOx, and with incomplete mixing of fuel and air, they produce more soot than gasoline engines.

Best of both worlds? Homogeneous Charge Compression Ignition. Just like a gasoline engine, you mix the fuel and air in the port - but you rely on compression to light it off. You leave out the throttle plate, and control the timing of ignition by how much boost you use, how warm the incoming mixture is (there’s a heater in the intake), and how rich the mixture is. With no throttle plate and lean combustion, you get good efficiency; with premixed mixture, you get low emissions.

But as you might imagine, it’s a bitch to control, and power output can only be controlled within a relatively narrow range. Solution? Reactivity Controlled Compression Ignition. You inject gasoline in the port to create a mixture that’s too lean to ignite on its own when compressed, and then let the cylinder draw it in and compress it. When you finally do want to ignite it, you inject a tiny bit of diesel fuel into the combustion chamber, which ignites quite readily, and in turn lights off the lean gasoline mixture. Now you have efficiency better than a diesel, the emissions of a gasoline engine, and the controllability of a conventional spark ignition system.

I think something like this may be among the most promising technologies to mate with hybrid drivetrains.

This will suffer from the same problems as old-school Wankels: tip seal durability, and a high surface-area-to-volume ratio that results in high unburned hydrocarbon levels in the exhaust.

This isn’t just a conventional 1-cyl engine like your lawn mower: it’s a free-piston engine. With no crankshaft, the piston motion is controlled only by cylinder pressure and actively-managed electromagnetic forces. Power is extracted by the same coils that are managing piston motion, so you don’t even have a separate alternator. Fewer moving parts means less weight and less friction, so yay - but that piston motion is tough to control. If you screw it up, the piston slams into the valves, or into the other end of the bore. someday this could be a good technology to mate up with RCCI, providing the best of both worlds: efficient production of mechanical work through RCCI, and efficient delivery of that work to the wheels/battery through the free-piston mechanism.

These could be good bridge technologies for passenger cars in the next 10-20 years, and they may eventually find a permanent home in long-haul vehicles for which regenerative braking isn’t a huge advantage - but my hope is that battery tech continues to improve to the point that most city-dwelling vehicles are pure electric.

That’s a really neat idea, Machine Elf, and I thank you for your reply. I take it the RCCI engines do require two sources of fuel?

I was thinking you’d have the fuel tanks and the monopiston engine/generator all in one compact module. Ideally it could be removed and replaced with an additional battery + charger of the same size and output voltage. So city slicker cars would have that space in the car be empty, suburbanites might have the extra battery, and people using their cars for revenue pickup or long distance driving would have the engine. And yeah, it you could get that kind of amazing efficiency on a tiny 1 cylinder engine, that would be pretty awesome.

59% indicated efficiency on E85/diesel! Wow! (read your link)

Yeah, you’d design your generator engine to run at just 1 speed at the highest efficiency speed. That way you get the most efficiency out of it. Wonder if you could air cool it, so you can leave out the coolant and coolant channels and all that other complexity. 59% efficiency on a production engine is incredible. It’s also so efficient that at current gasoline and diesel prices, I wonder if you even would save money plugging a vehicle driven by such an engine into the wall…Let’s see.

I don’t know what this geek fascination is with “pure” series hybrids. there are operating conditions where mechanically coupling the powerplant to the final drive is more efficient, which is why GM designed that capability into the Volt (causing the nerds to lose their minds.)

Exotic engine designs have almost all gone nowhere. not because their concepts don’t work, mind you, but that they’re competing with piston engines which work nearly flawlessly for hundreds of thousands of miles with minimal maintenance needs. Most novel (i.e. non-piston) engine designs have serious problems with sealing, lubrication, durability, and so on. Even the one which has been worked on consistently for decades (the Wankel/rotary) still has problems with those things.

Turbines are a non-starter since they only work efficiently at peak load, and for an equivalent amount of shaft horsepower they still are less efficient than piston engines. And as with pretty much any engine design, as you scale them down in size they get even less efficient.

Nationwide, average gas prices : $2.419. Diesel, $2.534. 33.4 KWH/Gallon, gas. Diesel, 37.95 kWh.

59% efficiency, let’s assume that the charging chain is the same after. (which isn’t quite true - some of the energy when the engine is running would go directly to the drive motor and skip the battery)

Assuming a 50-50 mixture, a gallon each of gas and diesel gets you 71.36 kWh of energy, which is converted to electricity at 59%, leaving you with 42.1 kWh remaining. National average electricity prices are 12 cents per kWh, so that’s $5.05 worth of electricity.

The fuel cost you…drumroll…$4.953.

Now, each hour of engine runtime does put wear on the engine, and it’s probably several thousand bucks, especially with the additional valves and plumbing for a second fuel tank, etc. On the other hand, it’s not unfeasible to make the engine modular in a way that allows it to be easily and cheaply repaired with aftermarket parts from China. Essentially it has the metal part with the monopiston and the integrated control coils, an electric control board with the FETs and control processor, the fuel assembly, the emissions/muffler assembly, a valve assembly, and so on.

Still this has some definite promise. The prospect of an engine that is as cheap as plugging into the wall (more or less) would be enormously useful on certain classes of vehicle. Semi-trucks, pickup truck EVs, long range capable EVs, Autonomous EVs used as taxi services would all benefit.

It would also have the bonus of having a very high fuel efficiency on the window sticker.

A Tesla is 284 watt-hours per mile. Since 2 gallons of fuel give you 42.1 kWh, you get 74 mpg running on fuel alone.

The simplicity and elegance of it. Especially pared down designs like single piston designs. No driveshaft or transmission or any of that stuff. The Prius has double the number of drivetrain components a comparable Toyota sedan has. Electric generator would run at a constant speed and not have at transmission or a design requirement for torque. That liquidpiston design has a video of someone assembling the engine from a handful of parts on a table - about 15 total parts. A fraction of the complexity of a piston engine.

Also, the modularity of it. You could ship the car and add the engine later, or remove the engine and put a battery where it goes, assuming the manufacturer did a modest amount of work to make the dimensions reasonable.

To be fair, you’re probably right. None of these exotic engine designs - even if they are better if fully developed- can compete with the vast amount of refinements and vast supply chain for conventional piston engines. Even if series hybrids do eventually become dominant - I would actually suspect that the existing manufacturers of piston engines would come up with a way to make an appropriate piston engine using their existing factories and supply chains for a lower price-tag.

Also, a series hybrid with a large battery - 2/3 the capacity of a Bolt or Model 3, essentially - would rarely burn fuel and thus not need a particularly efficient or long lasting engine.

Does this sell cars?

Any saleable configuration has to be tested and certified for emissions, fuel economy, and crashworthiness. Your proposal adds a ton of possible permutations which have to be certified, and that Testing is quite expensive.

It’s 3 configurations. Nothing there, a battery there, an engine there. Don’t exaggerate the problem, existing manufacturers ship models with and without sunroofs, etc.

Elegance may not sell cars, but “74 mpg on pure fuel” and “no range limit” and a low price would sell a ton.

Especially to the target audience - operators of these vehicles as taxis, and semi-truck operators. They would dominate the industry if they were more efficient for the same cost (for semi trucks) and no need for a charging stop, and same deal with taxis.

If you have a car which offers both a manual and an automatic transmission, both configurations have to be certified separately. We’re not talking about sunroofs here, we’re talking about sizable power train and energy storage subsystems. I do this kind of thing for a living, I’m not “exaggerating” anything.

My point is that it’s not any worse than it was when they have several configurations, and *is *a drop in the bucket to the manufacturer’s costs to ship a vehicle.

So yeah, you are in fact exaggerating the problem quite significantly. You probably don’t work in the C-suite at a major automaker doing the planning for a vehicle, so your experience doesn’t apply. (I don’t either, but it means your experience card is worthless in this discussion because it’s the wrong experience)

Wow, if that’s where this is going then I’m just going to disengage.

So your fragile ego is so hurt from a criticism of “a couple million bucks spent crashing a couple extra cars variants is nothing compared to the billions it takes to develop a car design” that you’re just going to “disengage”. Enjoy your safe space.

Wankels were never better… just different. And their few advantages went out the window when they had to have a camshaft and valve train added to make them emissions-compliant.

Neat tech. The Mercedes C-111 was an amazing beast. The earliest RX-7s were jewels. But in the end, rotary engines were just… different for different’s sake.

Wankels have high power density (power to weight/size) at the cost of poorer thermal efficiency and worse emissions. I usually point out that the last RX-8 got worse fuel economy than the Mustang GT despite being 600 lbs lighter and having just over half of the horsepower. Add to that inherent oil consumption and shorter service life.

I’m not sure that was true for the later models. Originally, the valveless, largely emissions-less rotary was a very powerful and lightweight engine that had no piston equivalent and was wonderfully matched by the light, agile RX-7 chassis. The original rotary was competing with big, sloppy, low-efficiency engines designed in the 1950s.

But down the road, adding the valve gear, chamber insulation and emissions control engineering, coupled with the fantastic improvements in piston engines (to where 100HP/liter was common, not something from an F1 garage), made it pretty much a wash.

I’m not aware of any valvetrain added to automotive Wankels. Mazda relocated the intake and exhaust ports to the side plates with the Renesis, but that’s about it. But the low thermal efficiency comes from the large effective surface area of the combustion chamber (heat loss to the coolant) which is an inherent shortcoming of the design.