REX Means Range
Over a year ago, we reported in Best Approach to Electric Car Range-Extension Still Evolving on the various approaches to range-extender (REX) technology. One thing seemed clear: when you add a REX to an electric vehicle, you gain a lot of advantages. You can downsize the batteries, which makes a car cheaper and lighter. (Even with downsized batteries, it’ll be capable of covering most folks’ daily driving needs of around 40 miles in electric mode). Yet, you still enjoy a large total range, since the REX jumps in whenever you want to visit your grandma, or go on a vacation. A REX turns a green commuter car into a Swiss army knife: zero-emission most of the time, but versatile when you need it.
It wasn’t clear though, what kind of range-extending technology is best. Some like the conventional Otto-cycle internal-combustion engine (ICE), others say Wankel is best, and some adventurous engineers envision a turbine engine.
In the meantime, a lot has happened, but nothing has been decided. The Chevy Volt uses a conventional gasoline ICE in front. Audi’s A1-Etron will have a small, free-spinning Wankel unit in the back. And Jaguar says their turbine-in-the-middle solution has great potential.
So the REX ecosystem is open. And now, a small, Swiss company has leaped right into the middle of this ecosystem.
Swissauto is a name you wouldn’t know, but they have been making turbocharger units for numerous car manufacturers for decades. They also make racing engines for Formula One a well as for go-carts. I met Swissauto’s CEO, Beat Kohler, at a conference in Switzerland, where his company’s co-operation with the Mindset EV start-up was announced. Here’s what we discussed.
Q: Herr Kohler, what is your system’s unique selling proposition?
Kohler: It’s good that you start with the most important question! Our USP is that we have put the engine and the generator into an integrated, compact, lightweight unit. It weighs around 36 kilograms, which is only about a third of what the most lightweight engines being used in cars now weigh.
Q: Wow, just the transmission of a Lexus LS 460 weighs over 100 kilograms. But won’t such a light system be a bit weak in the knees?
Kohler: We designed this one specifically for lightweight cars such as the Mindset, which hits the scale at about a metric ton, and we also tested it for use in a 1245-kilogram VW Polo. Generally, we believe in downsizing: a REX doesn’t have to be big, heavy or powerful. With a displacement of 300ccm and an output of 26kW, it may seem low-power, but our calculations show that due to the high efficiency of electric propulsion, this is enough to enable battery-hold mode (i.e. not charging the battery, but keeping it stable) until a speed of around 120 kph.
Q: The Swissauto REX is a one-cylinder, four valve, internal-combustion unit. So, it’s probably loud and causes a lot vibration, right?
Kohler: It’s funny: everybody thinks a Wankel is quiet, as is a turbine -- if they only knew. We analysed the NVH (noise, vibration, harshness) characteristics of various engines and found that both the intake and exhaust noises from Wankels and turbines were rather critical. It’s easy to beat them. In terms of vibration, an Otto-cycle engine like ours is in principle not so good, but ours is fine due to the fact that it has a balancer shaft. In addition, a small engine has little mass, and that translates into less vibration.
The nice thing about a REX is that when you plan it from an early stage of a car’s development, you can completely isolate it. You need zero mechanical connection between the REX and the rest of the car -- only electrical cables. So in practical terms, it is easy to make the engine noise level lower than the regular driving noises, and thus practically imperturbable.
Since the unit can be isolated from the rest of the car, you can shield it in sound-deadening material, and cut vibrations by using a soft engine mount. It’s really quite elegant in principle.
Q: And you claim it is highly efficient?
Kohler: Yes, at 35%, it has the highest efficiency of any REX we know. It always runs in its efficient working range with an almost-open throttle; output is only controlled by engine speed. It is a free-spinning, low-friction engine that uses, quite uniquely, roller bearings. Reaching a gas consumption of about 80 MPG is easy for our REX.
Q: How does your system compare in terms of cost?
Kohler: A one-cylinder ICE is cheaper than a Wankel or turbine engine -- that’s for sure.
Q: People worry about the reliability of range-extender engines. Turning an engine on only every two months sounds like a recipe for trouble.
Kohler: Actually, that’s one more thing we like about our REX. Since it has roller bearings, it always remains well-lubricated. Even after a rest period of a few weeks, it can immediately be revved up to 4,000 RPM without any damage. Oil changes will hardly ever be needed.
Q: So, your REX is king of the range-extender field?
Kohler: Let me put it this way. Car makers tend to define themselves primarily as engine-making companies, and it takes a leap in faith to try a smaller, simpler approach to engine design. We hope we are making a contribution.
blog comments powered by Disqus