Hybrid vehicles will use an internal combustion engine system that is coupled with an electric propulsion vehicle and the most popular is the gasoline electric car. While the gas electric hybrid car is most popular you can also find diesel electric hybrid cars as well as others that use other alternative fuels. This incorporation of the electric engine makes the cars more fuel efficient and has better performance.
These hybrid vehicles will have a conventional engine that can use gas or diesel and then contain a large battery as well as an electric motor. So the wheels actually use both the gas and electric motor. There are several different types of setups as they can be used in parallel, in a series or in different combinations. An example of a parallel hybrid is when both engines are connected to the wheels and driveshaft. When the series hybrid is used the conventional engine is used to create electricity which is then used to power the wheels. A combination would work by having the electric motor running the rear wheels but the front is powered by the gas engine.
Diesel electric systems are advantageous for long term driving. These engines and transmission have much higher efficiency and they do not wear as much when operating. Diesel also has high torque which can provide really great mileage. Diesel hybrids can also use bio-diesel which has even more benefits as it does not have to use any petroleum products. You can mainly find these diesel hybrid power trains in commercial vehicles though there are no passenger cars that use this type of technology. BMW, PSA Peugeot Citroen and BMA have all developed prototypes for diesel electric hybrids.
There are limitations to the gas electric car as you can mix up to 15% bio-ethanol in with the gas but then you are limited to the fuel you can use. The main difference between the gas and diesel hybrids are that with gasoline the fuel and air are mixed and then compressed, whereas with diesel only the air is compressed. This compression causes heat. So diesel runs cleaner and you also get more energy from diesel than gas because diesel contains much more BTU.
It is thought that gasoline hybrids were developed first because gasoline still is the main fuel used in countries with large auto markets. If diesel was the common fuels then these hybrids might have been the first developed. Which is the best will completely depend on what type of driving you do as diesel is best for long distance and gas is better for shorted differences.
While using diesel sounds great, the amount of money needed to actually produce the diesel engine in the hybrid is more expensive than the gasoline engine. So ultimately the upfront costs will be more expensive. Also diesel can be more expensive so the savings from using a diesel hybrid are not greater then a gas hybrid. The amount of taxes between diesel and gas are different which puts diesel at a disadvantage again as the demand for diesel is also on the rise.