Revealed – how the hybrid car "works"

The hybrid Toyota is a well known and claimed fuel efficient car. We all know its somehow got a battery and an engine.  But what is the idea? Why does this make it more efficient?

Essentially the Toyota is more efficient (well a bit) than many similar cars, because the engine operates on what is known as an  Atkinson* cycle, rather than the normal Otto cycle for petrol engines.

In case you are interested we explain how this differs from a normal car engine later.  But the point is that by fiddling around with the engine valve timing ( the valves regulate  when  in the rotation of the engine,  the air is admitted to the engine,  and exhaust gas is let out ), you can make an ordinary petrol engine (Otto cycle) quite a bit more efficient.

The problem is, this seriously lowers the power output of a given size (and therefore weight) of engine.  This means the acceleration is poor – there is less power to accelerate more weight.

The Toyota Prius  gets round this by having a battery and a motor / generator on board.  When the driver wants to accelerate hard, the feeble  engine is assisted by the energy stored in the battery driving the motor.

The battery is recharged by using the motor as a generator to slow the car (- regenerative breaking) or using the engine to top it up when it is not working hard at accelerating but cruising.

So the regenerative breaking, which saves some of the energy otherwise dissipated as heat in the breaks can be re-used which improves the mileage in towns.

But contrary to what you might suppose, the engine is more efficient than the equivalent Otto engine even when cruising, so mileage will be improved even for driving outside town.

The downside is that whilst the whole thing is on paper more efficient, you have to carry a heavy battery and motor around, and this eats up some of the savings.

The result is a car that performs as well as an ordinary Otto cycle, but a bit more efficiently, but is more expensive.

If you want real fuel economy, you are probably better of with a diesel.

The car certainly isn’t a partly electric car even if it can be charged from the mains – without any petrol the car will only travel about 1 mile – which shows how irrelevant the battery is in terms of using mains renewable power to substitute for petrol.

Fred Starr / Dave Andrews

*Atkinson cycle – this is an ordinary Otto cycle engine which operates on the well known four stroke cycle (suck, squeeze, bang, blow) but with the inlet valve closing considerably delayed compared to the Otto.  This has the effect of decreasing the amount of air compressed by the engine considerably and less energy is thus used to compress it, and more energy can be extracted during the expansion phase, since this arrangement allows the exhaust gases to more fully expand than in the Otto cycle.

In a turbocharged engine, the Atkinson cycle is for some reason known as a Miller cycle – a number of manufacturers e.g. Wartsila and Cummins make such  a Miller cycle gas engine because it has higher efficiency but lower specific output.

See Wikipedia for more info.


  1. Reply

    My Letter published in The Engineer in July 2005


    In reply to Mike Antonis’s letter; Mike may well be right about the possibilities of semi-hybrid technology but to dismiss the Prius technology out of hand is a little premature. Using the spectre of reported stalling is also underhand, even if true – how many IC power trains failed in the early days of motoring.

    Driving under varying conditions, I normally I get around 61mpg, however, I do concede that if you are driving flat out on the motorway the gain, if any is marginal – the technology is not magic but we need it even if it evolves, which it undoubted will – the electronics and drive will be needed for a fuel cell vehicle.

    I have driven a Prius for 30,000 miles and I have just written the following letter to Toyota – if there is room to include it.

    As a convinced owner of a Prius for well over a year I commend the following approach to marketing, which I submit now after a 12-hour journey from Cornwall last weekend and reading a letter on road charging in the Independent today, page 36, which states that congested roads add 50% more pollution than uncongested roads

    I feel the Toyota Hybrid vehicles are being undersold as vehicles that are more ‘fuel efficient and less polluting’. Whilst they are, this approach leaves Toyota a hostage to fortune as the pollution is purely emotive and the mpg obtained depends on the driver.

    A driver new to a hybrid vehicle will tend to drive it as a conventional model and will compare the bold mpg figure on the dash with an ideal figure from the manual for their previous car. This will in many cases lead to disappointment as evidenced by comments on Prius egroups.

    In essence the Prius is a ‘quality’ vehicle in that it aims to maximise the ‘value added to society’ from its creation, use and disposal.

    What does this mean in plain English? Well it means that the fuel and pollution savings are far more than is first apparent for the following reasons

    • In its creation the Prius uses the Toyota Production System created by Taiichi Ohno, this is recognised worldwide for its effective use of resources – known as lean manufacturing in the West. Whilst this attempt to minimise the embodied energy in the car does is not immediately reflected in the cost to the driver as they are paying extra for the technology, it certainly can be marketed as an extra feel good factor.

    • In use the mpg figure is misleading as this does not change when the vehicle is stationary. In my journey from Cornwall, although it took 12 hours, the fuel used was the same as the normal six-hour journey (actual reading 61.1 mpg). In a conventional car the engine would have been running for the total time and emitting pollution for the total time.

    • This was great but the vital point that I have not seen mentioned is that the engine was only running for a fraction of the 12 hours, with the attendant saving in mechanical wear and tear. In fact, although I have a car that has 30,000 miles on the clock, the engine has only done, perhaps 20,000 miles – all under the control of the system, with no strain placed on it by me. Over the life of the vehicle this must be reflected in considerable repair and replacement costs (hopefully!)

    • As the engine is not running when stationary (AC permitting) personal comfort is also enhanced in standing traffic.

    • Predictive driving also reduces considerably the wear and tear on the braking system.

    • Finally in respect of disposal at end of life, Toyota have strategies in place for this and this point should arrive later due to reduced mechanical wear and tear and vibration; especially if Toyota starts to sell the service, mobility, its Hybrids provide rather than the vehicle. This is probably more appropriate to second owners – as first owners tend to buy image.

    These are personal gains but the ‘value added to society’ from the demand side management and reduction in energy used from fossil fuels is considerable and will increase greatly as the proportion of hybrids grows and more of the standing traffic is composed of hybrid vehicles.’

    For over a hundred years we have ‘educated for industrial and environmental decline’ (Correlli Barnett – The Audit of War) and the western economies have created a ‘compliance and risk averse’ business environment, even turning Taiichi Ohno’s work on the Toyota Production System into the Hocus-Pocus of lean manufacture. Rover should have been poised to lead the automotive industry into the future – we can only blame ourselves.

    Derek Deighton
    Trailblazer Business Futures

  2. Reply

    Dear David and others

    Ed Marshall, in his recent paper for the Newcomen Society, points out that the Prius is just the latest variation on the “Atkinson Cycle”, invented in the 19th Century.

    The Atkinson Cycle delays the opening of the inlet valve on a normal four stroke engine.This permits the engine to work at a very high compression ratio, enabling it get towards diesel engine efficiencies.

    Combined with variable valve timing, this enables the Prius engine to maintain an efficiency of around 35% from an output of 15KW, up to the full power of about 42kW. Even when the engine is delivering only 4kW, the efficiency is still 25%!.

    A conventional four stroke engine runs at about 15% efficiency at the lower outputs needed for normal around town motoring.Even on the motorway the engine, in a noraml car will be running at about 40% of peak

    The downside, of the Atkinson cycle, is that because the amount of mixture delivered to the engine is restricted, the power output of the Prius is pretty mediocre by modern standards.This will reduce acceleration.

    Ed Marshall suggests that this is the main reason for the battery and electric motor.These supply additional torque to supplement what the engine cannot provide.

    Accordingly, if the petrol runs out, you are unlikely to get home on the battery! The wonders of the hybrid car are therefore something of a myth. The good fuel consumption is just due to clear thinking about the shortcomings of the four stroke cycle, and doing something about it. But it would have been a bit hard to get this through to the average motoring correspondent

    I have the suspicion that the great improvement in automotive diesels, and natural gas fueled IC engines for CHP, is that, when turbocharged they operate on a form of the Atkinson Cycle. Here, the late closure of the inlet valve, which would reduce the compression ratio, is counteracted by the boost pressure given by the turbocharger compressor. Within the engine, the negative work of compression is reduced, while the positive work of expansion stays the same, so efficiency is increased.

    Could Dave Andrews or someone who knows the CHP engine
    business comment?

    Ed’s paper can be bought from the Newcomens Society website and is published in the Vol 79/No1 of the Int Journal for the History of Science and Technology.

    Best regards

    Fred Starr

  3. Reply

    Miller cycle

    The Miller cycle, is the same as the Atkinson cycle ie delayed closure of inlet valve, but with turbo charging. This enables the turbocharger to force more mixture / air into the engine, eventhough there is little time for the charge to enter. Cummins / Wartsila make such an engine.

    Dave A

  4. Reply

    I am not a Prius fanatic, I just see it as one more iteration in the development of personal mobility that allows development of more than just fuel economy – controls, regenerative braking etc.

    This is tempting providence, but ours has done 92,000 miles now and, touch wood, all we spent is on the 10,000 services and two or three tyres. I have a Peugeot Partner estate and this has already had new brake shoes and an exhaust at half this.

    I just feel there are wider issues than just fuel consumption, such as noise and air quality in built up areas and the aggregate national reduction in consumption if all cars had cut outs when stopped.

    It’s generally a pleasant car to drive at normal road speeds but the mpg is falling off slightly after five years, down from 64 to 58 if I’m trying. To get this we would have needed a much smaller car with greatly reduced flexibility re. grandchildren etc.

    As I said in the letter, we must move to selling mobility as a service if we are to reduce its ‘resource intensity’; the idea that 3+billion cars can be fitted into what I call ‘the first law of sustainability’, that in a resource constrained environment, goods and services can only grow at the rate at which they can be dematerialized, is but the dream of fools.

  5. Reply

    Dear Fred,
    The key point about the Atkinson cycle is that the compression ratio is different to the expansion ratio. The engine has a very long stroke which allows the hot gases to expand more fully than they would in a normal petrol engine. This is more thermodynamically efficient and allows a low exhaust pressure, also reducing the exhaust noise.

    On the inlet cycle, the full length of the stroke can’t be used. The valve inlet timing has to be modified so that the compression ratio is only ‘normal’. It is restricted by the knock sensitivity of the fuel and any whizzo ‘swirl head’ design that can improve things.

    The down side is that because the stroke is longer than normal, the cylinder block is bigger and heavier than a normal car. There is thus a trade-off of increased engine weight against reduced exhaust weight and fuel efficiency.

    Superchargers can pack more fuel into any engine, thus saving on weight but may create their own problems of requiring yet more inlet valve timing tinkering to stop knocking.

    There is a trade-off of fuel efficiency against complexity and reliability.

    Bob Everett

  6. Reply

    Bob Everette’s description is another way of expressing the points I have been making about the thermodynamics of the Atkinson cycle.

    However, I would like to add that, by late closure
    of the inlet stoke, the effective compression ratio on the inlet stroke is kept at around 6/1 to 8/1 ( I guess). This reduces the compression work and also reduces the likelihood of the engine knocking. But on the exhaust stroke the expansion ratio is 13/1 so more work in generated.

    I suppose, because the so-called hybrid car only works with a Prius type engine, and is really there to provide the short terms acceleration which the engine cannot provide, there has been little interest in hybrids from other manufacturers.

    I did not know about the Miller cycle and its applications to both natural gas CHP and large scale diesel, in which the compressor of the turbocharger does part of the work in compressing the mixture.So thanks to Dave Andrews for this.

    This may explain the revolution that has occurred overthe past 15 years with CHP engines running on natural gas. The efficiency of these was quite poor at around 32%.This was inferior to a gasoline engine of the same capacity.

    I was surprised during a recent visit to Turkey to hear that their latest CHP engines were running at 39%. These figures for 1MW output natural gas engines have been confirmed from another source.

    This means the newer engines are offering fuel savings of 20%. Presumably there is a downside in that the exhaust gases are cooler than onthe older engins??? But despite what William Orchard thicks electcity is fas more valuable and useful than heat.

    Fred Starr

  7. Reply

    Need to be careful on how efficiencies are quoted….The ISO standard is to use LCV and an engine quoted at 39%, LCV would in fact only have a real efficiency in common sense HCV terms of say 10% less – about 35%.

    The LCV is based on subtracting the energy which is needed to vaporise the water since this cannot be used to provide mechanical power – but it can of course be used to heat say a swimming pool if the exhaust gas is condensing. (I’ve done it.

    The limit on the power of a gas engine is the exhaust valve temperature, so maybe the Atkinson / Miller cycle helps here.

  8. Reply

    Dear Dave

    TOPIC points Hydrogen, CHP, Hybrid cars, Engine Thermodynnmics

    All my efficiency figures that I gave are LHV, but I agree that this not the most rational way. The argument is that the LHV figure removes the fuel effects from effiency statments.

    Actually it doesn’t. When a gaseous fuel of any type is burnt in a gas turbine or internal combustion engine, power output and efficiency are down compared to operation on liquid fuels.

    The loss in power is easier to explain since the volume of air going through the engine is reduced. That is a gaseous fuel occupies more space than a liquid fuel so there is less room for the air. Since air is needed to burn the fuel, it becomes necessary to reduce the amount of gas going into the engine. That is the energy going into the engine is reduced.

    The effect is worst of course with low calorifc value fuels like hydrogen or producer gas.Hydrogen gets the double whammy, since, in addition, there is such a big difference between the LCV and HCV

  9. Reply

    Message flagged Friday, 24 February 2012, 19:28Dear Matin and All

    Today’s hybrids are best referred to as ” battery assisted Atkinson cycle machines”. The Atkinson cycle permits the IC enjoy to maintain high efficiency at low loads, that is for around town driving. The operation of the valves in the Atkinson Cycle allows a very large expansion ratio during the power stroke, whereas the compression ratio during the compression is held at a lower value, but still optimised for maximum efficiency. In the Otto, or normal four stroke cycle the two ratios are the same, hence the exhaust gas leaves the engine at at about 3-4 bar pressure wasting energy. Conversely at low powers in Otto cycle, the cylinder is not properly filled in compression stroke.

    The shortcoming of the Atkinson cycle is that at high power outputs, the airflow into the engine is effectively strangled, and since a major selling point for car manufacturers is power and acceleration, the straight Atkinson cycle is of little interest. This problem is overcome in hybrids by the “valve timings” and “valve lifts£ being mechanically changed when the driver wants more power. The engine then begins to work on the Otto cycle.

    The actuation of the valves to new settings takes time. During this period the battery supplies the power that the driver immediately expects. So the battery is only in use for a very short time, and does not need to have much capacity.

    The need for high power also undermines Martin Quick’s s very interesting suggestion about using weaker valve springs. Such springs would benefit both around town and 70mph motorway fuel consumption. Without stiff springs it would be impossible to rev car engines to high revs…….For unsupercharged engines power output is 8-10 times engine speed in 1000s of rpm times capacity in litres. So a 2 litre engine at 6000 rpm would have an output in the 96-120 hp.

    Fred Starr

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