Hello! Welcome to this edition of our series on micro turbines! Today we’ll be covering micro turbines and the roles they play in the automotive world.
“Big wheels keep on turnin’…”
Now here’s the real question, when you see that lyric which song do you think of first? Having gotten that stuck in everyone’s head, let’s get on with today’s topic: micro turbines in cars.
I mentioned in the intro to the series that when I think of micro turbines my mind immediately jumps to turbochargers like those used in reciprocating engines seen in cars, trucks, boats, and small airplanes.
They are, in essence, the same, but also different. For example, a turbocharger uses exhaust gas from a reciprocating engine to drive a compressor to pull more air into the engine, while a micro turbine drives a compressor to pull air into a combustor and then also drives a generator to create electric power.
Additionally, a turbocharger can run at over 100,000 (!!!!) RPMs, and in some applications like a road car, will constantly be at different rotational speeds. Now this isn’t always the case, in boats/ships or aircraft that operate at (relatively) fixed engine speeds while cruising, the turbocharger will run at a constant RPM; which is similar to how a turbine generating electric power (such as a micro turbine) would run.
History: Where did micro turbines get their start in the automotive world?
Micro turbines were introduced into the automotive market in the 1950’s after World War II. Notably, Chrysler and General Motors tried to capitalize on the relatively new jet technology that became the newest means of propulsion at the end of the Second World War. Unfortunately, these machines were commercial failures. As we’ve discussed in our entry on micro turbines in aerospace, these machines have a very narrow power band and very poor low-end torque; both of these are qualities you don’t want in an application where operating conditions and loads are constantly shifting. If you’re curious about this, you can read our blog here that covers this topic more in depth.
Toyota revived the idea of a micro turbine as the primary mover in the 1980’s, however as you can guess by how many are on the road today, this didn’t really pan out. So what were Toyota, General Motors, Chrysler, and the others trying to achieve by using small scale turbines as a replacement for reciprocating engines? Well, despite everything written so far about the serious disadvantages of having a micro turbine be the primary propulsion system in an automobile, there are some advantages we shouldn’t discount:
Advantages to Having a Micro Turbine as a Primary Mover in Cars
- – Very few moving parts compared to a conventional reciprocating engine.
- – Longer maintenance intervals compared to a conventional reciprocating engine.
- – Very high power to weight ratio when compared to reciprocating engines.
- – Potentially lower emissions.
- – Fuel diversity; gas turbines can run on a variety of different fuels.
Disadvantages to Having a Micro Turbine as a Primary Mover in Cars
- – Very poor low-end torque – this can lead to poor fuel economy and wear on a turbine as drivers would likely put the turbine under sharp heavy loads from a standstill.
- – On a related note, they have poor throttle response, which is really bad in situations where you need to merge onto a highway, or you’re trying to win that drag race.
- – The entire industry would have to retool to manufacture/service turbines for the automotive world.
- – Turbines require certain rare metals that can withstand the heat from hot exhaust gas, and as a result can be very expensive to buy.
- – Turbines have might tighter tolerances than reciprocating engines, making them more delicate and susceptible to failures from adverse external conditions.
- – Small turbine components can be difficult to manufacture properly, and will the drive up the cost of a vehicle per unit.
- – Since the automotive industry’s infrastructure is centered on reciprocating engines, turnaround times for turbine engines would be higher than average.
By now it should be clear that although there are some advantages to having a micro turbine be the primary mover in an automobile, the disadvantages almost certainly eclipse those benefits.
So how can micro turbines be used in cars and trucks? Well in a similar fashion to their aerospace counterparts, there are a few different ways in which micro turbines can be applied in a car. So here are a few different ways they can be applied in cars and trucks:
In this case, the internal combustion engine is kept vague, however we can assume in the context of this article that the engine is in fact a micro gas turbine. So as we can see, there are several different ways to make use of a micro gas turbine in an automobile/bus/truck.
From left to right on the top we start with a conventional battery-powered electric vehicle, with no combustion engine. On the top right, we have a series hybrid electric vehicle, where the internal combustion engine is coupled to the transmission and drive line of the vehicle as well as a generator paired to a battery. This generator and battery also act as a motor to drive the turbine and in turn the transmission and the vehicle. In the middle row, on the left, we have a different series hybrid electric vehicle. In this application, the turbine has no physical connection to the wheels of the vehicle, and instead, powers the battery under a more stable and constant load in the same manner to a power generation turbine.
On the right in the middle row, we have a parallel hybrid EV. In this configuration the micro turbine and the electric motor/generator work together to power the wheels, but without one supplying power to the other. In this case, the motor and the turbine are each taking turns propelling the vehicle similar to how we see conventional hybrids currently. Typically under a certain speed, the car is propelled by the battery-powered motor, and then the turbine/internal combustion engine takes over propulsion over that speed threshold. The same methodology is used for the series hybrid EV on the bottom, except the generator receives it’s power or “work” from the turbine instead of using the motion of the wheels and transmission to give it power.
And lastly, at the bottom we have a series/parallel hybrid electric vehicle. This example is similar to the previous application that we just looked at, except you’ll notice that the battery-powered motor will also allow the micro turbine to provide power back to the motor and battery in addition to driving the wheels of the vehicle.
So where are micro turbines being used/prototyped in the automotive world?
Well, there’s a few interesting cases I’d like to show you. The first one comes from a joint effort by Kenworth Trucks, and with Capstone Turbine corporation. In September 2017, Capstone, which specializes in building micro turbines and Kenworth, a trucking industry giant came together to test the viability of using a micro turbine as a source of power and paired to a motor/generator as opposed to a reciprocating engine driving the transmission and the wheels.
The test proved successful, as the micro turbine-powered truck was able to successfully propel the vehicle using the hybrid powertrain. While doing so, it consumed less fuel and produced less emissions than a conventional turbo-diesel reciprocating engine.
Another, flashier example, came in the form of the special concept car the Jaguar C-X75. It was planned to be a very limited run car developed in conjunction with the Williams F1 racing team.
The onboard powerplant for this James Bond villain-mobile was 2 70 kW range extending gas turbines utilizing diesel fuel supplying electricity and range extension to the 580 kW electric motor. On electric power alone, the car could go a paltry 68 miles; with the micro turbines, however that range went up to a colossal 560 miles. That’s farther than most cars will go on a single tank of fuel! The car’s top speed is an equally impressive 205 mph with 0-62 being dealt with in less than 3.5 seconds, thanks to the powerful 580kW electric motor. Sadly, production was halted in 2012, but the idea of the Jaguar C-X75 lives on!
Micro turbines have been a part of the automotive world in one form or another since the Second World War. The idea of using a micro turbine as a primary mover has been explored, although has not shown very promising as a concept put into practice. However, using a micro turbine as part of a hybrid powertrain to supply electricity to the batteries and in turn, the electric motors, is very much a feasible option, has been explored, and will likely continue to be explored.
On the next revolution…
Next time, we’ll have a look at micro gas turbines in marine transportation, and where they may be useful to supplying power with better fuel economy and emissions.
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