How do hybrids perform so well? MPG AND MPH

Yes, it's a fraction, but every bit of energy saved by regen is an improvement in economy. For example on a recent outing to Stowe in our plug-in hybrid, about the limit of its 10kWh battery EV range, it claims to have regenerated 2kWh.

There is also the possibility of using the engine more efficiently, eg:

2) Optimise the engine for a small range of speeds, eg using Atkinson cycle, as it doesn't need to be smooth and flexible over a wide range of speeds

nib

What would be the practicalities of having an EV with a small highly efficient engine permanently running a generator to charge the battery, thinking really small perhaps even something like s Sterling engine?

Pretty sure it’s been done (with a normal IC engine) but improvements in battery tech is quickly rendering most hybrid solutions redundant, for those with the capability to charge at home at least.

There’s still a big issue to be resolved for those who can’t though.

Tim

BMW i3 has a detuned aircooled bike engine which runs optimally to charge the battery at 15kW IIRC

Theo

I have that. Apparently invented by Toyota, but Ford use it too (although they changed the layout of the two motors). The gearbox has relatively few moving parts.

(I have a Ford and my wife recently bought a Toyota)

The engine in a hybrid only really has to be powerful enough to run the car at top legal speed on the flat.

Any time you are NOT doing that, it can charge the battery, as can braking.

As long as the battery is capable of getting you up the longest climb in the country at 70mph WITH the engine, that's all it has to do.

AS others have pointed out, this is an engineers dream. Design an engine that is as efficient as possible at what - 30bhp or so - and then add battery, 80bhp electric motors and regenerative braking and robert is yer mums bro.

What you WILL find of course that at 70mph constant speed, its no more efficient than a fuel engine would be.

In town driving, most of the energy your car's engine produces gets turned into heat by the brakes. A constant 30MPH on a flat road uses very little energy, it's the traffic lights, road humps etc. which use most of it. The Prius was an early hybrid, and probably most of them were bought by taxi drivers.

Note that the much-maligned Sinclair C5 with its modified washing-machine motor used regenerative braking.

It wasn't maligned. It really was that crap.

-- Renewable energy: Expensive solutions that don't work to a problem that doesn't exist instituted by self legalising protection rackets that don't protect, masquerading as public servants who don't serve the public.

Indeed. Most of the stuff boosts efficiency when you aren't travelling at a constant 70mph on the flat. When it is, it's just a regular small petrol engine with ok but not outstanding efficiency.

OTOH every journey starts and ends at 0mph. It's the time spent between there and 70mph that it improves. If you're on the autobahn all day long that time is relatively insignificant and you might look at a diesel or a full EV. But if a decent chunk of your driving is spent going less than

One other thing though: because the cars have a relatively small engine for their size - ie you can fit a 1.5 litre engine in something that would normally have a 2.0 petrol or a 2.4 diesel (or bigger) for equivalent power

- you can get better efficiency than you would ordinarily expect on that size of car.

The downside is they're not so good at towing, which is when you might work that 2.0 or 2.4l engine harder for a long period, rather than relying on it for short bursts of power that can be provided by the electric motor.

(regen does help a bit when towing heavy things, although some will be lost in trailer brakes)

Theo

Interesting point - although the trailer brakes should dissapate pretty much all of that potential energy if they are operating correctly.

On 07/09/2025 11:35, Theo wrote: > When it is, it's just a regular small petrol > engine with ok but not outstanding efficiency.

The serial hybrid (range extender) has a slight inefficiency compared with a manual gearbox in that if the engine is running most of the electrical power will bypass the battery and go directly to the motor, that excess which goes to the battery suffers a round trip loss of about

My diesel car uses about 1.6kWh of heat energy to run a mile on an average motorway run, I think an EV of similar size and weight would manage a mile on 0.25kWh does this mean my car is only 15% as efficient as the EV??

BTW cost wise my mate's Honda civic 2008 challenges the diesel at about

Diesel is about 48 kWh / gallon, so 30mpg?

Most hybrids are series-parallel hybrids in that the engine can charge the battery and/or drive the wheels directly. When you are driving at 70mph the engine can drive the wheels for maximum efficiency, rather than converting to electricity and back again. That's why serial hybrids are more intended as a secondary power source in an EV which is ideally running on battery much of the time.

On bigger things like locomotives then going via electricity makes more sense in terms of matching engine RPM to traction requirements, which is why many are diesel-electric (and they are now starting to have batteries, although that's mostly for shunting etc).

Not sure about your 1.6kWh/mile figure. Here:

On those numbers the average diesel is 28% as efficient as the average EV.

Theo

Most diesel engines can turn around 30% of input heat energy into usable power. The maximum theoretically possible depends on the maximum temperature within the engine and the exhaust temperature. (See Carnot Cycle, Thermodynamics, etc.)

An EV is essentially 100% efficient, although the thermodynamic losses took place at the power station.

So, your figures of 0.7087 etc look right.

On a motorway it would be nearer 45%.

Ex of Fuel duty and with only 5% VAT my heating oil is around 6p/kWh...

They do better, 45% or more

About 90%, plus regen

Some car engines have been using the Atkinson cycle engine, and using the electric part (and a magical CV transmission), to fill in the performance gaps. That should improve the fuel economy.

The most powerful three cylinder engine, uses 95mm pistons, a big:small twin turbo (for lag), and makes 600HP. There is no camshaft on that engine (the valve timing can all be set in software). They don't all have to be boring.

The internal combustion engine, isn't all that efficient, so when you do make one closer to the ideal, it makes a difference. The "fleet average" for cars, shows that a lot of the engineers aren't doing any work. They're stamping out "muffin tins" and most of the effort goes into "teh profits". A design cycle is a "cost reduction", a thinner axle, a less robust alternator, and so on. And the exact same engine as the last generation of "pig".

A sign of efficiency, is when the ICE car has poor cabin heating capability and can't extract enough waste heat from the engine to do the necessary heating. On some of the BEVs, the waste heat from the liquid cooled electric motors, goes towards the cabin heat, via the all singing and dancing heat pump and plethora of valves.

If you look at our diesel bus transit fleet, there is a ton of waste heat coming off the back of those, and also available for heating the interior by some means. No Carnot Cycle involved there :-)

I was reckoning on 10.15kWh per litre diesel and the car does did 16.3 miles per litre on the last fill, mostly 50 mile journeys on motorway and local roads.

I'm not sure how these EV miles per kWh are calculated, it should be by kWh supplied from the charger but is it?