I'll see what I can remember - I'm a software engineer, not a mechanic! ;-)

The Atkinson cycle was originally developed with a special crank system - see this link (warning - large animated GIF). But the engine manufacturers have worked out a way of doing roughly the same thing using the valves to set how much air goes in and out. A page about the Toyota Prius drivetrain... and another one about our Ford one. Incidentally, Toyota can give it as many fancy names as they like to try and say they invented it, but in fact they just licensed it off a company called Aisin, the same as Ford have.

The big deal with the Atkinson cycle is that it's very efficient. However, it's got really bad power output at low revs, which means it wouldn't work for normal cars. But low revs on a hybrid-electric is handled by the electric motor, not the engine - the engine doesn't turn on until the car's doing 20-25mph, by which time it can perform just fine. And electric motors put out maximum torque at zero RPM, so that gives it a really good boost. Ford's stated aim for this is 3l V6 performance with a 2.3l engine, and with 35mpg economy instead of sub-20mpg. From latest showings, it looks like they'll get it, too.

Controlling this setup is an absolute nightmare though. The only thing that doesn't have a computer choosing what happens is steering. The brakes have a separate solenoid that can stop the driver's pedal-press engaging the friction brakes to use regenerative braking instead (converting the vehicle's momentum back into electric energy to store in the battery). The powertrain has no gears at all - there's just a 3-way planetary gearbox with the engine and two electric motors on it, so the gear ratio from engine to wheels is entirely controlled by what you do with the electric motors, which basically means it's all software-controlled. The gear lever is just a sensor telling the software what to do, there's no gears physically engaging or anything.

Given how much software there is, and the level of control we have over what the vehicle does, there's always going to be some fun with that. I've personally experienced some really good bugs in this, and I've heard of some even better ones (including an early prototype deciding to engage full forward acceleration when the driver was reversing - new underpants please! ;-). Hopefully none of those bugs make it through to production though! That's why this takes so damn long though - it's so new, and there's so much to get done, and you just can't take the chance on getting it wrong. On something like this, the risks are much more like the risks in an aircraft failing than the risks in a normal car engine going wrong, given that the driver doesn't have the option of braking or even de-clutching if the computer won't let them.

Graham.