Featured Mazda competes for thermal efficiency against Toyota with Skyactiv-Z

The friction is hardly a reason, but I guess every little bit helps.

But then @bwilson4web points out a good point (I'm guessing is his point), that it might be possible for a free piston engine to move the piston up and down in a more efficient cycle other than just following a crankshaft round and round at fixed stroke speeds.

Something similar has been done with Mazda's rotary range extending engine, being disconnected from the wheels allows control over speeding up and slowing down the rotor at specific moments, in this case allowing for control over intake and exhaust timing, which normally isn't possible in a rotary engine,.

 

Certainly a poor mechanical design can do worse than a Carnot cycle. With rare exception, there is no "free lunch". The rare exception is the delayed close, intake valve of the "Atkinson" engine combined with:

• POWER ON - MAX EFFICIENT MODE
  • bank excess power in battery/capacitor
• POWER OFF - MAX EFFICIENT MODE
  • withdraw banked power from battery/capacitor

In the big picture, EVs are efficient because the grid thermal cycles generating electricity are so efficient and the vehicle batteries store the variable power needed to drive. So the spectrum:

• variable ICE - least efficient, 10x $ per mile
• variable diesel - more efficient, 7x $ per mile
• variable Toyota hybrid - more efficient 5x $ per mile
• variable BEV grid - more efficient 2x $ per mile
• variable BEV solar roof - most efficient 0x $ per mile (fusion powered by Sun)

Bob Wilson

 

Many companies/ ventures are pushing the boundaries of engines/free piston devices/linear generators into new realms not thought possible.

In this case the power/energy generator is called a linear generator- it generates power by making and breaking chemical bonds.
These devices are actually currently in use in commercial applications in mainstream companies like Kroger.

This illustrates why car manufacturers/people movers must be exploring a whole array of different technologies if they want to remain a vital part of the people moving industry - energy generation/power manufacturing is not standing still it is moving forward.

"It’s January 2030 and your electric heat pump is warming the house while your electric car charges in the garage, all powered by solar panels on your roof and by wind and solar generators at your local utility. It doesn’t matter that it’s been raining for two weeks because your utility is tapping into ammonia produced with last summer’s sunshine. It’s consuming that ammonia in a linear generator."

This New Breed of Generator Can Run on Almost Any Fuel - IEEE Spectrum

 

Most of the conversion losses for a series hybrid are taking place in the beginning, with the chemical>heat>kinetic>electrical changes. To minimize those loses, you want the engine running at peak thermal efficiency. An engine can have an 'island' of efficiency in its operation band where that peak is maintained for a range of output. That island won't be large enough to cover the full demand a car sees in operation. The problem with the i3 REx's hybrid efficiency was in it not running at thermal efficiency peak, and instead having output follow the driver's demand and varying engine output as you desire.

Before the Recession, Audi was working an A1 E-tron PHEV that was a series hybrid with a small rotary engine. They initially ran that engine at a single speed for best efficiency. That got on the nerves of some drivers, but they settled on 3 fixed speeds and not full variable speed.

A linear engine will reduce the kinetic>electrical losses with having less mechanical parts to move in the system. That still leaves the electrical>chemical and back in the battery at a fraction of the loses in first making the electricity.