Help me understand the synergy drive.

The engine runs or is programmed to run by the ECU's at it's most efficient speed/power output. This can mean that at a given load it can be more efficient to power the car along and charge the battery.

Other cars have to use a large engine running at a fixed RPM (for given mph) that is a very inefficient fuel wise for that speed.

A close look at the workings of the power split device will give you a better understanding of how this works. For the engine in the Prius to power the wheels MG1 must either be braked or powered. If it is allowed to spin freely the car is effectively in neutral.

Take a look at ( Toyota Prius - Power Split Device.) for a very informative veiw on the subject with animation.

John (Britprius)

 

When I was a teen in the 70s, I remember reading where putting a 5Hp engine on a small car like an MG was all that was required to move along on flat level ground. They experimented even then with electric generators and batteries. Saying that every time you coast or stop at an intersection the 5Hp engine running is wasting fuel. To capture that energy into a capacitor or battery saves it for when you accelerate.

I had a buddy once [who was a Reactor Operator] he built a car that used hydraulics and store the energy in an accumulator. That can be a very efficient system.

Running a 100Hp engine with all the fuel it consumes, when you only 'need' that much power for 5% of your driving is wasteful.

Remember the engines they made with 8 cylinders, but that allowed you to isolate how many cylinders actually get fuel? If you only need the power from 2 cylinders, then why send fuel to the other 6? If you only need the power of 4 cylinders, then why send fuel to all 8, when you could turn 4 off?

A small engine is all that you need for 80% of your driving. You may need a 'boost' on occasion, so it is nice to have a traction battery online. And when looking at that 80%, honestly for half of that you do not need what you are running. How much power do you need when your coasting? How much when your idling? Better to store that energy for the next time your climbing a hill.



As to losses from charging a battery. Yes there are losses. A hydraulic system works better and has much less loss. But nobody is going to mass-produce such a system. The first time that any mass-produced car with hydraulic accumulators charged to 6000psi gets in a wreck and the tanks rupture. Wow! Imagine the law suits. Better to stick with batteries.

 

For a full brake down of the subject go to :-Gen2 Prius technical discussion and in the yellow box at the top click on "introduction to prius power flow.

John (Britprius).