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What is the most efficient technology for powering a car?

Updated on February 10, 2013

Introduction

As a first step, you should read my hub: http://hubpages.com/hub/What-is-better-diesel-or-petrol-engines. That covers off the issues and differences between petrol and diesel cars, so I won't repeat the information here.

Recently, there has been much press and discussion about hybrid cars. I own one, so I feel knowledgeable in talking about them. Are they the answer? In short, the answer is "No, not in their present form". Unfortunately, battery power is not very effective for cars, and we will explore this later in this hub, looking at purely electric cars.

We will also take a look at what I think could possibly be the answer - turbojets in cars! Read on.

Tell me about hybrid cars - what are they and how do they work?

Hybrid cars are so-called because they use both electrical power and fuel power. There are two types. One type drives the car purely with electric motors, and uses the petrol engine to drive a generator to re-charge the batteries, or to provide additional power to the drive motors, when required, such as for rapid acceleration. This system is used to extend the range of an otherwise purely electric vehicle - and we'll cover those later.

The other type, which is the system used in the car I own, uses both a petrol engine and electric motors to drive the wheels of the car, and switches between them, or uses both, as the driving situation changes. You can accelerate very slowly up to about 40mph using just the electric motors, but acceleration is slow, and if you are in traffic, then you are likely to be holding it up! Put your foot down a bit and the petrol engine kicks in, and works either alone, or with one or two of the electric motors to drive the wheels. You get maximum acceleration when the engine and the two motors are all working together. On my car the two motors produce a maximum power equivalent to about 1000cc of petrol-engine capacity, so the car has a smaller engine by that amount than it would otherwise have to achieve the same power.

The car uses regenerative braking, which means that when you hit the brakes, the car takes some of the energy of motion of the car, and turns it into electricity, by using the two motors in the car in reverse, as generators. This also increases the effectiveness of the brakes over standard brakes. The car also saps a small amount of energy from the car as it travels along normally, by using what is effectively extremely light regenerative braking to top up the batteries whenever you take your foot off of the accelerator pedal. Regenerative braking is one of the main keys to the greater efficiency of the car over a standard petrol car. This is where energy is reclaimed for re-use to supplement the energy generated by the petrol engine.

Hybrid cars: how much more efficient are they?

In practise, you do not get a lot of additional miles per gallon from the regenerative breaking. In a half hour journey you might re-claim about 600Wh (Watt-hours) of energy from the regenerative braking, but when you accelerate you typically use 100 to 200kW of power. 600Wh is 216kWs (kilowatt-seconds), so at an average acceleration of 100kW, your energy regenerated over the last 30 minutes is only going to provide you with 2 seconds of energy not coming from the petrol engine. That's not a great deal. On average, our car does about 25mpg (miles per gallon). It is an SUV, so that might be quite efficient for an SUV, and we do not drive it particularly carefully to save fuel, we drive it quite hard. On one long journey we got 38mpg.

The greatest efficiency actually comes when driving through urban areas, when the car is stopping and starting all of the time, because the system switches off the engine whenever you stop. This can enable you to get up to around 60mpg.

So, in conclusion, the hybrid car is not such a great breakthrough, only providing a small increase in efficiency over conventional petrol-engined cars.

What about purely electrical vehicles?

As we've seen with the hybrid car, electricity stored in batteries is not great for driving cars, because they use so much of it: 100 to 200kW when accelerating; and if you compare this to an electric bar heater, or fan heater, which might be 1 to 3kW, or a light bulb, which is only 60W, or 0.06kW, then you are beginning to see why electrically powered vehicles, with the electricity stored in batteries, are very difficult to build with a decent range and decent acceleration.

Some have been produced with great acceleration, but they do not have much range, and some have been produced with a good range, but lousy acceleration, and then they require re-charging for many hours (maybe as many as 8).

The other thing to consider with electric vehicles, is that they are not necessarily more efficient in overall energy usage than petrol-engined cars. The electricity has to be generated in the first place, and this is done in power stations in the UK mainly through burning fossil fuels to generate heat to create steam to power huge electromechanical generators, and/or burning fossil fuel directly in turbojet engines to power generators. The electricity then has to be distributed to the point of consumption for the vehicle to plug into, to charge its batteries. Overall, the maximum efficiency of the best combined cycle electrical generation is about 60% (before distribution). When burning fuel directly in a petrol engine to generate the power, the efficiency is only 20%, as most of the energy generated goes out through the exhaust as heat. However, if electric vehicles became very widespread in their use, then more power stations would need to be built to provide the capacity to charge them all, and when you take into account the energy consumed in constructing a power station, and the environmental effects of building new power stations, this starts to look less attractive than sticking to petrol engines to power cars.

What they do achieve, is that they move the problem of exhaust gases off of the streets in our urban areas to power stations. Obviously, if you live somewhere where you can take advantage of hydroelectric power generation, then the argument sways back the other way, but in the UK the only real alternative to fossil-fuel-powered electricity generation is nuclear power, and that has its own issues. Renewable energy sources such as wind, wave and solar power are not going to be able to generate a significant proportion of our electrical energy needs for decades to come, especially if electric vehicles significantly increase that demand.

Then what about fuel cells?

As we have seen, the issue with electric vehicles is producing enough electricity to get both power for reasonable acceleration, and good range. The fuel cell has been considered as an option, because it is a kind of "battery" which can produce a great deal of power. It works by combining hydrogen and oxygen to create water. This reaction can be used in a fuel cell to produce usable electricity as its by-product. One great advantage is that the only waste product is pure water.

The big draw back from a fuel cell is that you have to carry cylinders of compressed hydrogen and oxygen in the car. This is fine until the car is involved in a major accident. If the hydrogen cylinder ruptures, or worse still, both the hydrogen and oxygen cylinders rupture, you have a major explosion on your hands, killing all in the immediate vicinity. This is why fuel-cell driven cars have not materialised.


What about turbojet engines?

The reasons that a turbojet, like we see powering jet aircraft, has not been used to power a car to date are two-fold. Firstly, we cannot have cars on our streets blasting out red-hot gases from the rear at high power, so we can't use them like we do in an aircraft.

Secondly, if we thought of using a smaller jet engine to power a gearbox and drive the wheels directly, the problem is that a jet engine doesn't work very well at low speeds, and in fact wouldn't work at all at the low speed required to accelerate from a standing start. Perhaps some kind of electromagnetic clutch could be used to achieve the low starting speed from a jet engine running quite fast, but then a lot of power is being wasted, and the engine would not be very efficient.

However, Jaguar came up with a revolutionary new idea, which could have changed the face of future motoring. It is a concept car called the C-X75 and used tiny jet engines to produce electricity to drive electric motors which power the car. In prototyping, the jet engines were dropped, as the emissions were too high at start up. This is a huge shame, as I thought this idea showed great potential. Worse still, unfortunately, due to the global economic downturn, Jaguar suspended production of the car in December 2012, although they have said that they will sell two of the working prototypes at auction to collectors. Find out more at: http://www.autoexpress.co.uk/jaguar/61846/jaguar-c-x75-production-cancelled and at: http://www.topgear.com/uk/car-news/jaguar-c-x75-will-not-be-produced-2012-12-11 .

How did Jaguar's concept jet car work?

The car used two of the smallest turbojet engines ever made - just 70cm across at their intake fan, about the size of a pint beer glass - and weighing just 35kg each. These ran at a constant high speed, where they are most efficient (about 30% efficient, thus 10% more than a petrol engine), and were used to power generators to charge batteries.

The car itself was driven by four 145kW electric motors, one on each wheel, powered from the batteries, or from the batteries plus extra electrical power directly from the generators for greater acceleration. The 780bhp produced enabled it to accelerate from 0 to 60mph in 3.4 seconds. The car produced just 29g/km of CO2 and with a 60 litre fuel tank had a range of 600miles. So this car was faster, cleaner, and with greater range than petrol cars. The car would also run for about 60 miles without the turbojets, i.e. as a purely electric car, so it could have been used in this mode for short journeys.

What a shame that this never made production as a jet-powered car, although Jaguar say that they learned enormous amounts during the research and design phases and have patented more than a hundred new technologies as a result. I hope that this project is re-instated when the world economy recovers (whenever that may be!) and we see jet-powered cars in the future.

Conclusions

Electric cars are becoming more common, and a lot of research is going into them. Hybrid cars are a way of combining electric powering with a conventional petrol engine to get greater range than you would get from a purely electric car, and avoid the need for re-charging, but they do not bring much improvement in efficiency.

What is needed to make electrically powered vehicles become the norm, is a more efficient, and safe, way of generating large quantities of electricity in a car, and Jaguar may have come up with the next significant step with their twin micro-turbojet system. This pushes up fuel consumption economy to 45mpg in a car which competes on speed and power with cars doing about 10 to 15mpg. If, in the future, they can develop the technology to overcome the high emissions at start-up, this could still be the technology powering the cars of the next few decades, enabling the next level of efficiency to be reached.

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