Why “in-line” hybrid engines aren’t used in cars?



I recently learned that train engines have been using electric motors powered by diesel generators since at least the 30’s, which surprised me. It made me wonder why cars or at least trucks and busses don’t do something similar, and why “parallel” hybrids are the only type of hybrid engine widely available.

I assume it has to do with the size difference, but what specifically? Needs to carry too much liquid fuel? Generator or electronics can’t be miniaturized? Or is it a purely economic problem and not a technological one?

In: Engineering

I’m not 100% sure but I think that Opel Ampera 2012, Chevy Volt and BMW i3 used or use this configuration. Edit : But they also have batteries too so combustion engine basically only extends range. Trains use it because they are very heavy and clutch would be quickly destroyed. There are also diesel hydraulic trains.

The reason train locomotives have been using diesel electric in this way is because they produce too much torque for any gearbox that could fit to handle. Diesel electric is able to produce a lot more torque then a traditional gearbox but there is a lot of power loss though the generators, wires and motors. In cars the torque is not that big of an issue for a gearbox. So the electric motor in a hybrid car is not primarily there to replace the gearbox but instead to provide additional performance through the use of batteries.

It used in car in combination will batteries.

The advantages of diesel-electric in the train are that you need to have high torque a low wheel speed. You need that in a train in a way that is not needed in a car. A car torque converter with automatic gearboxes or clutches with automatic works fine.

A diesel engine, generator, and electrical engine will weigh more than just direct drive in cars

Diesel eclectic is common in ships. There is space-saving if you have electric wires instead of long propeller shafts. You can put the engine in the middle of the ship for better weight distribution.

It also makes it possible to have multiple engine-generator where all is not used at the same time. You can have multiple diesel generators for low-speed operation. For high-speed operations, you can add a gas turbine generator. Gas turbines are efficient if you use all the power from the and small for the power they provide. So warships and cruise ships have them when they like to go fast. Some cruise ships put them up in the smokestacks.

So with multiple engines and lars space, they have advantages.

The result is that is it not efficient for cars to just have diesel-electric drive.

The idea can be used in cars if you combine them with batteries. You can then use the batteries to provide power when lost is needed like if you accelerate and store energy in them from regenerative braking. The batteries can also be charged from an outlet

The internal combustion engine only needs to be powerful enough to provide enough power when you drive at a constant speed. You can operate and the most efficient power level.

This is called a [Series_hybrid](https://en.wikipedia.org/wiki/Hybrid_vehicle_drivetrain#Series_hybrid) and a [Chevrolet_Volt](https://en.wikipedia.org/wiki/Chevrolet_Volt) or [Nissan E-power variants](https://en.wikipedia.org/wiki/Nissan_Note#2017_model_year_update_%E2%80%93_e-Power) is a common example.

There is a drawback with this designer. You need more components in them compared to just using electricity or just an internal combustion drive. A pure electric will be more efficient if you just drive the distance of its batteries to reach. Just internal combustion engines cost less.

Because it wouldn’t offer any benefit in a car. Conventional hybrids are designed so that the electric motor can supplement the power from the internal combustion engine and reduce the fuel consumption of it. An ICE driving a generator and then using that power to drive an electric motor would just be adding extra inefficiencies into the process–the only advantage you might get is that you can have the engine running constantly at its most efficient speed rather than revving up and down.

It’s entirely an issue of weight, complexity, and cost. It’s totally doable, and honestly advisable, but it ends up being too expensive to justify it in the current market. That will likely change in the future, though.

Are you referring to a configuration where the gas engine exists only to charge the batteries and never transfers torque to the drive wheels? A few cars actually have used that style of hybrid. The Fisker Karma was one. I might be wrong, but I think the Chevy Volt is another.

Trains use diesel-electric because they need the high torque a large electric motor provides. Their top speed is similar to a car’s highway speed, but they are thousands of times heavier than a car, so acceleration using just diesel motors would be almost impossible.

Some plug-in hybrids can operate this way, using the engine to charge a battery and just running the wheels off electricity. But the power still has to go through the transmission, so you don’t gain anything from it. And cars are light enough that a gas engine easily provides enough torque to get moving.