The ideal hybrid car.

Throughout my teenage years I spent a large chunk of my time fixing up motorcycles and studying the science of engine tuning. One of the problems with 4 stroke engines was that the valve timing could not be changed. Systems have now been introduced that modify valve timing a little as engine speed and torque requirements change but the flexibility is extremely limited. I used to wonder if magnetically operated valves would be the answer. When I learned more about the subject I discovered that the acceleration forces on valves are huge and available magnetic materials are far too weak to deliver those forces. I therefore dropped the subject until recently.

It is possible for hydraulics to deliver the forces required. There is now a fair bit of literature on the subject of electro-hydraulic valve actuation (EHVA). A quick review I did recently revealed a number of ideas that looked somewhat simplistic. The key issues did not seem to me to be addressed, but I did not pursue it far because I had a better idea explained below. However, if you are interested in my EHVA ideas please do get in touch.

The main goal of variable valve timing is to deliver a more flexible engine that is efficient at low power but can also deliver exciting performance when required. By thinking about the wider picture I realised that the best solution is outside the engine. Model aircraft have undergone a radical change and by using relatively recently developed magnets and batteries electric craft now out-perform their combustion engine powered predecessors. The power density of modern electric motors is quite remarkable and beats that of combustion engines. Therefore, why not use this fact to build a better hybrid car?

The recently introduced Tesla electric car has shown that lithium batteries, modern electric motors and electronic power control can deliver an exciting level of performance. Gone are the jokes about boring milk float performance! The cost and weight of the lithium batteries is still an issue so my idea is to reduce the quantity needed. Most journeys are short and would only require a small battery bank. This is well recognised which is why numerous companies are developing hybrid cars. I offer my thoughts here in the hope it might just boost the process along.

Battery and motor technology has now reached the stage where it can deliver sufficient power density to get drivers interested. Since these systems deliver almost unlimited flexibility and speed of response it makes sense to deliver power to the wheels with a purely electric system. A combustion engine of some type is still the best option we have for delivering range because the energy density of diesel, petrol(gasoline) and paraffin far exceeds that of batteries. Therefore, while a system that uses an engine to drive a generator that charges a battery that powers an electric motor may seem complicated, it is starting to look like the best way to achieve excellent fuel economy and good performance.

Most effort on battery development has been on increasing the energy density so that they last longer. Progress has been slow with this goal but in the meantime great advances with power density have been achieved. Batteries are available that can be charged and discharged in 15 minutes or less. Many battery powered car developers have ignored this feature of batteries and continued to concentrate on battery range extension. Disregarding the reality of the direction battery progress has taken has wasted time and funds.  A battery capable of lasting just 20 to 50 miles (30-80km) could still be powerful enough to deliver an exciting ride. This is far enough for the majority of journeys taken in the average car. For those less common longer journeys a hydrocarbon fuelled power source can extend the range. Most hybrid cars announced so far use a single fairly standard engine but there are important advantages to using several power sources.

Fuel cell

The best source of power to charge the batteries is the grid, but the batteries in a small pack would run low on energy rather quickly. It would often be wasteful to start up a full-sized engine to charge them because the journey might not last much longer. Therefore the first range extending power source should therefore be a tiny single cylinder engine, or a fuel cell. The fuel cell option is explained more below. The engine would be tuned for maximum economy and would only have the power to get the car to town traffic speeds. The engine would be directly connected to a generator that doubles as the starter motor and it kicks in when the battery gets below a certain charge level. The engine is either off or running at full throttle at its optimum speed. Optimising it for a single speed and power output level will allow it to deliver far better fuel economy than an equivalent engine designed for flexibility.

Around town the small engine will keep the battery bank charged while delivering excellent fuel economy and pollution levels. For higher speed driving there will be at least one more engine, each driving its own generator. The power to drive a car increases rapidly as the speed increases. Speeds of 25, 50 and 90 mph might be achieved with engines of 3, 9 and 40kW. (50 mph with 2 engines and 90 mph with all 3.)

The bigger engines would be increasingly tuned for power rather than economy but all would be optimised for single speed operation. That is why I stopped thinking about my EHVA ideas.

The driver need not be aware of how each successively larger engine is started as the state of charge of the battery bank drops. However, the option of control would be useful to maximize fuel consumption. If the car was parked somewhere with no electric power and there was a long drive home one would want the smallest most efficient engine to carry on running to top up the charge. On the other hand, if there was mains electric available it would be more efficient to stop all engines immediately. Being able to change the point where the different engines cut in and out could also be useful, but a default setting would be important for the average driver who does not want to deal with that level of complexity. Integrated sat navs are becoming popular features of new cars and these could be used to help decide when to start and stop the different engines.

An alternative to the energy storage problem that is frequently proposed is to have the equivalent of a filling station where discharged batteries can be swopped for charged ones. One of the big problems is that many of these places would need to be built to supply the wide coverage required so this would take a huge investment over a long time. Developing the hybrid car discussed here would be much quicker. There would also have to be standardization on just a few types of battery otherwise we could get to a station and find they did not have our type in stock. I can also foresee problems with people leaving behind a good battery for charging and being given a poor one that does not last the journey and leaves them stranded.

The battery recharge stations could work in cities and busy highways but in smaller towns and rural areas I cannot see an alternative to the plug-in hybrid in the near future. As I discussed on my clean energy solutions page, aircraft are always going to need hydrocarbon fuel until a radically different propulsion system is invented. We are therefore going to have to crack the problem of the renewable production of these fuels. There is no fundamental reason why this could not be done; it just needs the ideas bubbling under the surface to be given sufficient support. These clean biofuels are what the ideal hybrid car would also use.

To get maximum economy there is the possibility of the smallest generator being a fuel cell (FC). Fuel cells are of interest because their potential for efficiency is unmatched by any heat engine. The big problem is that it has been exceptionally difficult to get their prices down to a level that is even vaguely economical for a car. Again, there has been an emphasis on developing a FC that can supply all the power needed by the vehicle. By using the more complex hybrid system proposed here we can make use of the strengths of a number of different systems and avoid their weaknesses.

The most efficient FCs operate at very high temperature but these have been routinely dismissed for transport applications because of the time needed to get them warmed up. However, using my multiple generator proposal there is only a need for a system of about 1 to 3kW and something that size could be hot all the time.

The FC would 'wake up' as soon as the driver opens the door and would continue until the journey's end. If the battery was not fully charged it would continue charging it after the journey's end but at a reduced rate to optimize efficiency. A well insulated solid oxide fuel cell can deliver over 90% efficiency at low loads.

To prevent the FC ever needing to be unused for extended periods it could be used to supply power via the charging lead. A 1 to 3kW FC could supply most of the needs of a house. By using a small fraction of its power most of the time its efficiency would be excellent and it would produce lower emissions than modern gas fired power stations.

An intelligent grid would allow a great deal of distributed power to be generated. The FCs in the cars would communicate with a central grid control computer and when extra power is needed they could help supply it. When there is an excess of grid power the FC would take a rest and any partially charged car batteries could be topped up.

High temperature fuel cells can be integrated with reformers so that they can run on the same fuel as combustion engines. There is therefore no need to deal with the huge difficulties of in-car hydrogen storage.

Turbine

Another thought about the ideal hybrid car is that the most powerful generator could be driven by a turbine. Turbines have excellent power to weight ratios, and low cost, but they are only efficient when operated near maximum capacity. Their limited speed range and slow response makes them unsuitable for direct drive to the wheels, but in a hybrid they would be ideal. They would only be needed for long high-speed runs so the slow start-up and limited flexibility would be no problem.

Having 3 completely different power sources in a car might at first seem excessively complicated, but more careful consideration reveals mainly advantages. Each would be a self contained unit. Put fuel in and electricity comes out. Therefore, each unit would be relatively easy to install and remove for servicing. The car would still be functional even if 2 units failed so reliability would be improved. In fact they could be made so easy to remove and replace that they could be removed whenever portable power is needed. For instance, if you were unable to park near your house you could take out the fuel cell and put it in your house and use it to supply the domestic electricity. Because each system is optimised for a special purpose the over-all flexibility of the vehicle would be far beyond the reach of anything on the market today. It would have sports car performance when needed, or extreme fuel economy when not pushed. The cost could be extremely competitive too. Modern manufacturing methods mean that complexity is no longer expensive. The smoothness and power of a modern V12 could be bettered using a hybrid with a FC, a single cylinder engine and a single turbine. The fuel consumption would be in a new league. The massive development effort to make the modern car engine all things to all men has resulted in an immensily complex unit, so this proposed 4-part hybrid could actually deliver more for less.

Inspiration.

This report about the development of a fast electric car by racing car engineers in Norfolk is a fantastic inspiration showing how new technology is waiting to be exploited.
www.renewableseast.org.uk/news-industry-detail.asp?ID=149
It illustrates how talented people given the right backing can create remarkable progress.

Others have been thinking along similar lines and some interesting product announcements have been made. Capstone Turbine have developed a micro-turbine that they claim delivers excellent fuel consumption. Some real figures would be interesting. MTT have also announced what they call a range extender turbine. It is tiny so it only delivers 16% efficiency. For the moment it appears piston engines are still superior at this power level. Their technology applied to a larger turbine, operated less often, would improve efficiency.  www.wilsonturbopower.com claim a much more impressive efficiency of 50% by going to 300kW and using ceramic blades. www.bladonjets.com are claiming 25% efficiency from their 70kW prototype. Jaguar’s C-X75 concept electric sports car uses 2 of them and they are claiming a typical fuel consumption of about 40 mpg and a top speed of 200 mph. They would have achieved better fuel consumption if they had used a single larger turbine but I guess Bladon jets are concentrating their development effort on the bigger market for rather less sporty cars.

The Lotus range extender is an engine optimised to run a generator at 3500 rpm. This I think is a mistake for hybrid vehicle use because it makes the engine unnecessarily large. Low rpm means it will last well but since the engine is only operated for a small proportion of the drive time it would have been better to have optimised for a much higher power density to allow more room for batteries, and a smaller weight penalty.

Page first edition 12/03/2009, last updated 1/11/2013.

Copyright © Robert Copcutt 2013