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Old 03-24-2005, 09:52 AM   #9
SaabJohan
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Re: Hybrids from an engineering standpoint

Gasoline-hybrid cars usually show a decreased fuel consumption similar to that one can get with a diesel engine.

A hybrid works best when driven in the city with accelerations/braking and standstills where the engine can be shut down.

A generator and a set of electric motors are a quite efficient transmission system, both works with high efficiecies which gives a high total efficiency. I would assume that the total efficiency of the system will be close of that we see in a mechanical transmission system. The highest efficiencies could probably be reached with one electric motor on each wheel hub, this would also allow a chassis control system to control torque at all driven wheels.

When a battery is charged and then discharged some energy is lost, but the benefits of the better use of the engine together with the energy recovered during braking will be much larger than the total losses.

As known, an otto engine is most efficient when working at high load, then we see efficiencies of over 30% while the same engine at part load may be just 15% efficient. This means that the mean efficiency of an engine used in a car is quite low, and the more powerful engine you have in relation what is needed the lower the mean efficiency will get. When driving at highway speed a typical car won't consume much more than 10-20hp, so fitting that car with a 200 hp engine is bad from an efficiency standpoint.

Today, with regards to cost, waste material from the car and emissions I would go for a diesel with a particle filter instead of a gasoline-hybrid, the exception being larger cites where the traffic can be slow or stand still for periods of time. The latter is also what for example Toyota Prius was designed for.

The hybrid technology can also be applied to compression ignition engines (the benefit is however smaller due to the better part load efficiency of the CI engine). If one builds a hybrid vehicle with a methanol fueled CI engine one will have a car that can beat the fuel cell cars in terms of efficiency for decades to come.

When it comes to store energy for short periods of time there are also lighter and more efficient options than batteries. In space flywheels made of carbonfibre are sometimes used. There have also been experiments of using a flywheel for energy storage in vehicles.
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