I drove back from uni the other day, and on the way I thought about different types of transportation. On this trip, for example, I only took back a few bags, but on the trip up, I had a carful of stuff. I also saw many different sizes and shapes of 'cars'- vans, lorries, little hatchbacks (like mine), family saloons, campervans and big 4x4s. These varied vehicle designs are made to do different things: a small hatchback is not designed to carry the same volume of stuff as a big 4x4, for example, but it is more fuel-efficient (as the 4x4 engine is bigger to cart around a bigger car). Most cars built nowadays are multipurpose.
In this post I'm going to talk about the different needs of the varied vehicles on the road today. These needs include range, top speed, and acceleration. Obviously, a city runabout doesn't need to have the range or top speed of a long range cruiser.
The most important part of the 'car' is the engine: what makes it go. Pretty much all engines out there on the roads are internal combustion engines, which use petrol or diesel to explode a gaseous mixture of the oil fumes and air, pushing down a piston, turning a crankshank, which turns the wheels. I'm not going to go into detail into how these car engines work, but see here if you are interested. This is all very well, and has been working around the world for over 100 years. However, these engines are very inefficient, by modern standards: 20-30% (from wikipedia). Compare this to a electric engine, for example, which has an efficiency of around 90% from batteries (source- this excellent page by the Tesla Motors Company, which has a lot of other information about efficiency as well) or about 60% from a hydrogen fuel cell (source) [*]. Add this to the fact that the fossil fuels used in the internal combustion engine are both producing greenhouse gases (CO2) and are running out, and you can see why it's worth considering other options for our transport.
[*]- Note that both battery-powered and fuel cell-powered cars use electric motors, but different methods to store the electricity (in the battery or hydrogen).
So what different types of vehicle are there? What designs do we need to use, and which type of power would be best for each?
________ Different types of 'car' _____________________
Small city car
The first type of car we will look at is a small city car or hatchback. This would mainly be used for short journeys, not carrying that much stuff and a maximum of 4-5 people. It also might be used for longer journeys on occasion. So, the main properties we are looking for in the small car is acceleration and efficiency, with perhaps range, refuel time and top speed not quite as important. I think that a good type of drive for this car would be an electric engine with batteries, with additional range-boosting energy recovery systems (regenerative brakes, for example). There are already quite a few of these "city runners" around, like the G-wiz or Megacity. However, these cars are often very slow accelerating, cramped and not very safe in a collision. A different method is being taken by a few of the bigger car companies- take a small petrol car, and replace the petrol engine with an electric one. This has been done to critical acclaim by smart and bmw (mini)[review of the mini]. I think that this is a better option than going for the "strip everything out and make it as small as possible" approach, as it requires less of a change from what we are used to driving. The reduced range of the bigger mini and smart could be compensated for by good regenerative technologies (brakes and so on).
Family (large) car
The next type of car to be examined is a large family or saloon car. This would typically be used for all types of journeys, whether it be taking the kids to school, going on a weekly shop or travelling long-distance. The main properties we want from this type of car are: range, top speed, refuel time and size (carrying capacity), with acceleration and efficiency perhaps not as important. I think that a suitable replacement engine for this larger multipurpose vehicle would be a hydrogen fuel cell.
Hydrogen fuel cell engines work by reacting hydrogen and oxygen together to make electricity (and water). The energy is stored in the hydrogen, which is currently quite a difficult substance to store- it's a gas at room temperature and pressure. There is much research into different methods of storing hydrogen, from metal hydrides to graphene. I am hopeful that we will, relatively soon, be able to store hydrogen safely and in large volumes.
There is another problem, though. Hydrogen is not a fuel, like petrol or diesel- it is an energy storage medium (think of it like a battery). It first needs to be "charged", i.e split into pure hydrogen from water (H2O). This is usually energy intensive, and if the energy used to convert the hydrogen is from fossil fuels, we haven't solved any problems. One possible solution to this problem is to use renewable, intermittent energy sources (wind, solar and so on) to convert water to hydrogen. This way, it doesn't matter (as much as electricity production) about the lulls and peaks in production from the wind not blowing or the sun not shining, as the rate of hydrogen will simply change. Iceland, for example, could produce hydrogen from its geothermal power sources and export it to Europe- the EURO-HYPORT project. See here.
Back to the family saloon- A good example is the Honda FCX Clarity. The hydrogen fuel cell engine is long range of 240 miles (compare this to my car- 300 miles or so), spacious, quick to refuel (similar timing to a petrol pump, and similar methods- see here *) and a top speed of 100mph (plenty seeing as the speed limit on UK motorways is 70!).
* This article also has an interesting self-sufficient solar hydrogen generation station.
Large van/truck
Another important and prevalent type of vehicle on the roads today is the lorry- a very large heavy goods vehicle. The important properties for powering this type of 'car' is range, power, and top speed.
We could return to the hydrogen fuel cell, with simply a bigger fuel tank and a more powerful engine. However, I believe it would be more interesting to discuss another fuel type: biofuels. These are a range of fuels derived from biological systems, whether it be alcohol grown from crops, methane from fermentation of waste, or ethanol from algae.
Biofuel-powered lorries would use internal combustion engines, so would require very little modification. Much of the methods for making biofuels use energy that would be otherwise wasted (e.g. methane from fermented waste), so this idea seems like a good one.
However, there are also quite a few problems with biofuels:
Firstly, they use the internal combustion engine, which as we saw above, is only 20-30% efficient, which is not good enough by modern standards.
Secondly, there are problems with fuel crops taking up space that would be otherwise used for food. This is a major problem, but not one I will go into great detail on. See here for more info.
Thirdly, biofuels are fossil fuels. Burning them will release CO2 into the atmosphere, so this is no real change from the current petrol/diesel engines!
I personally think that biofuel power is not a good alternative for transportation, as it is inefficient, politically problematic ("food vs fuel") and not really an alternative- it's just a way of making more fossil fuels quickly!
It seems, then, that hydrogen fuel cells might be a good replacement engine for the large lorries. They have certainly been trialled extensively for buses, which have similar power requirements to lorries (see here and here for bus trial details).
4x4
The final type of car we will examine is a 4x4. This is somewhat different a transport type, as it is designed to be used in more extreme environments- not just for on the roads. Therefore, it is important that it is reliable, long range and powerful. I think that the best type of engine for this type is an electric engine, possibly in each wheel, powered by hydrogen. This way the 4-wheel drive system could be managed to be efficient (not wasting energy turning the wheel when it isn't required to), reliable (as each engine is self-contained and can be repaired individually) and powerful (the in-wheel motors provide very high power outputs).
Examples of the in-wheel technology are here (e-traction direct drive) and here (wikipedia page with examples).
____________Conclusion_________________________________
From researching different types of engine, it is clear to me that electric engines provide a superior alternative to the internal combustion engine. They are far more efficient (60-90% as opposed to 20-30%), much simpler (only 1 moving part) and more scalable (can be made bigger or smaller for different power outputs- this is true also of the internal combustion engine, but the electric engine can be made far smaller).
However, there is a problem of how to store the electricity to be used by these new electric-powered cars. Battery technology is not as well developed as motor technology, so more research needs to be done in this area before a major change can be expected. Hydrogen, on the other hand, seems to provide much higher range but has it's own problems too- both in production (energy intensive) and storage (more research needed). I firmly believe that these problems can be solved to provide a new infrastructure of electrically powered vehicles which will be far better than our current petrol/diesel powered ones. In a similar way in which we look back at 1950s vehicles and laugh, we will be able to look back in 2050 and be amazed at how inefficient cars were 'back then'.
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