The Internal Combustion Engine

The Prius has a 1497 cubic centimetre (approximately 1.5 litre) displacement gasoline internal combustion engine which produces all power for the car.  Since the car cannot be plugged in, there is no other long term source of power and this engine must supply energy to charge the batteries as well as to move the car and power accessories such as the air conditioning, electric heat, audio, etc.  Toyota's designation for the Prius engine is 1NZ-FXE.  Some confusion is created by similarities with the Toyota Echo engine, which is 1NZ-FE.  The engine configuration is the same, that is four in-line cylinders, 16 valve double overhead cam with chain drive, cross-flow manifolds, pentroof combustion chambers, etc.  Not only is the displacement identical, but so are bore and stroke (75 by 84.7 mm).  It is quite likely that Toyota use the same basic block casting for both engines.  However, the unconventional features of the Prius engine make it so different from the Echo engine that it would make more sense to say that the Echo engine is similar to the Camry engine than to say it is similar to the Prius engine.  Simply put, the Prius engine uses the Atkinson cycle whereas the Echo uses the conventional Otto cycle, like the Camry.  In this topic, I intend to explore the Prius engine in more detail.  Even if you don't read through it, please treat any conclusions drawn about the Prius based on the similarity of its engine to the Echo with extreme suspicion.  I will use the Echo engine below to contrast with the Prius engine.

Conventional Internal Combustion Engines

The engine in almost every (gasoline fuelled) car you see on the road today is based on something called the "Otto cycle".  The operation of such engines is characterized by four "strokes" - intake, compression, combustion and exhaust - with the valves opening and closing close to the ends of the strokes.  A very good explanation of the Otto cycle can be found at HowStuffWorks.  The advantages of the Otto cycle are fairly good thermodynamic efficiency (conversion of the heat energy released when the fuel burns into mechanical work), an excellent power-to-weight ratio and reliability due to relatively simple operation.  Most improvements on the Otto cycle have the goal of increasing efficiency and/or reducing emissions.  Inevitably, power-to-weight and/or reliability suffer and the Otto cycle continues to dominate where the engine is the only source of power for the vehicle.

Sources of Inefficiency

To improve on the efficiency of the Otto Cycle engine, it is first necessary to understand where inefficiency arises.

Partial Power

A modern Otto cycle engine tends to be most efficient at 40% to 45% of its "red-line" r.p.m. and 70% to 80% of its peak torque.  At higher r.p.m., friction losses in fast-moving engine parts increases.  Higher torque is achieved using "fuel enrichment", which reduces efficiency.  At lower torque, the engine suffers most from what is termed "pumping loss" (discussed below).  At the efficiency "sweet spot", the engine produces around 40% of its rated peak power.  For the Echo engine, for example, the peak power is 108 hp, so it will be most efficient in the general area of 35 to 50 hp.

Ideally, then, we would like to size the engine in a car so that in the most common driving situations, we use about 40% of the maximum power the engine can deliver.  Unfortunately, such a car would not be able to accelerate according to our expectations and would not be able to climb hills very well.  It takes only about 15 hp to drive a car like the Echo at 65 m.p.h. on a level road and considerably less at lower speeds.  But if we gave the car a 30 hp engine, it would take more than 30 seconds to accelerate to 60 m.p.h. and would slow to 30 m.p.h. on a 10% slope.  So, the Echo has a 108 hp engine for acceleration and climbing hills.  This means that most of the time the power demand is well below the efficiency "sweet spot" and fuel economy suffers as a result.

The fact that automobile engines run for most of the time at a small fraction of peak power and hence well below the efficiency "sweet spot" is called the partial power problem.

Pumping Loss

The major cause of loss of efficiency at low power is "pumping loss".  How is an Otto cycle engine designed for a peak power of 108 hp persuaded to run at an output of, say, 10 hp?  The answer is that the flow of air into the cylinders is restricted by closing a "throttle" valve.  This forces the engine to drag the air through a narrow opening, creating a partial vacuum in the inlet manifold.  As the air entering the cylinder during the intake stroke is below atmospheric pressure, there is less of it.  A smaller amount of fuel is injected and the resulting smaller fuel/air "charge" causes the engine to run at low power, as desired.  But, as well as having this intended effect, maintaining the partial vacuum in the inlet manifold wastes energy.  As the piston moves down during the intake stroke, normal pressure below it and a partial vacuum above cause drag on the crankshaft's rotation.  This does also reduce power output, which is what we want, but at the expense of wasted fuel, which we want to avoid.  Note that cars suffer from pumping losses even at highway speeds.  The throttle is really only opened right up when accelerating or climbing hills.

Interestingly, diesel engines do not have this problem because there is no throttle.  Low power is achieved by simple injecting less fuel.  This is one of the reasons why diesel engines achieve higher efficiency.  This technique cannot easily be used by gasoline engines because the burn temperature becomes too high and damages the cylinder (see sidebar Lean Burn - Why it is Usually Avoided).

Expansion Ratio

The conversion of chemical energy into work (mechanical energy) in a piston engine centers around the combustion stroke.  The fuel/air charge burns rapidly and creates pressure due to the generation of hot gasses, mainly carbon dioxide and water vapor.  This pressure acting on the top of the piston forces it down, turning the crankshaft.  At the bottom of the piston travel, the exhaust valve opens releasing any remaining pressure.  The proportion of the chemical energy released as heat that gets converted to mechanical energy depends on the "expansion ratio", that is the ratio between the volume in the cylinder when the charge is ignited and the volume when the exhaust valve opens.  The higher the expansion ration, the more of the heat energy and pressure can be used to push the crankshaft around.

Unfortunately, in the Otto cycle engine, the expansion ratio is the same as the compression ratio, that is, the ratio by which the charge is compressed before being ignited by the spark.  There is an upper limit to the compression ratio beyond which the charge does not burn evenly and causes something called "knocking" which can damage the engine.  Some sources attribute this to "pre-ignition", which is the tendency for the charge to ignite spontaneously, before the spark.  The result is that an Otto cycle engine must be designed to avoid knocking for a given fuel octane rating and cannot make proper use of a large expansion ratio.

Friction Loss

As the internal parts of an engine move, they rub against each other and lose energy due to friction.  An example is the piston rubbing against the cylinder walls.  As power output and spin rate increase, the losses due to friction account for a larger portion of the engine's gross output.  This is why efficiency falls off above the "sweet spot".  Oil is circulated in the engine to reduce friction, but the primary goal is to reduce wear to an acceptable level.  Until recently, engine design did not go to great lengths to further reduce friction and as a result improve efficiency.  Ironically, friction becomes more of a problem as engines get smaller.  So, when we make an engine smaller to address the partial power problem, we give up some of the gain to increased friction losses.

Fuel Enrichment

Most of the time a conventional engine mixes air and fuel in the proportion that will burn up all the fuel using all the oxygen in the air.  This is called the stoichiometric mixture (see sidebar The Stoichiometric Fuel/Air Mixture).  For gasoline, the ratio of air to fuel is approximately 14.3 to 1 (by mass).  This means that the engine draws in 14.3 kilograms of air for every kilogram of gasoline it burns.  Modern cars maintain the correct mixture using a mass air flow sensor in the intake manifold and residual oxygen sensors in the exhaust.  If the air to fuel ratio increases, so that an excess of air enters the engine, the mixture is said to be "lean".  Unburned oxygen in the exhaust does no harm itself, but a lean mixture tends to burn hot and can damage an engine not designed for it.  The high temperature can also cause nitrogen to combine with unburned oxygen and produce oxides of nitrogen that contribute to pollution.  If the air to fuel ratio decreases, so that an excess of fuel enters the engine, the mixture is said to be "rich".  Unburned fuel in the exhaust contributes to pollution and, since this fuel was not burned for power, reduces engine efficiency.

Conventional engines are biased towards a rich mixture when power demand is high.  This makes sure every scrap of air drawn into the engine is used up to get the maximum possible torque.  The unburned fuel can be oxidized, up to a point, by the catalytic converter, but its energy is wasted resulting in lower efficiency.  The EPA is introducing standards to ensure that cars limit pollution under conditions of aggressive driving.  This is probably intended to make manufacturers do something about the increased emission of hydrocarbons when the engine enters fuel enrichment mode for high power demand.  Unfortunately, the easiest thing to do is to give cars larger engines so that they can meet the aggressive driving cycle without using fuel enrichment.  As a result, the car is less fuel efficient.

Strategies for Improving Efficiency

The Prius and other hybrid vehicle make use of a number of techniques to improve efficiency.

Engine Downsizing

The partial power problem is addressed in a hybrid vehicle by simply using a smaller engine.  The engine is not sized for the maximum required acceleration or the steepest hill climbs but for light acceleration and the kind of climbs that continue for long distances.  When full acceleration is required or a short, very steep hill must be climbed, the car calls on the electric motor to supplement the engine using power stored in the battery.  The battery can be recharged from the engine when power demand later decreases.  A car of the weight of the Prius would normally be given an engine of around 110 hp peak power.  Instead, it has a smaller engine of only 70 hp.  This puts the engine's efficiency "sweet spot" closer to the sort of power demand that is encountered most of the time during cruising and traffic following.  The battery can supply an additional 26 hp when needed to bring the total available power to almost 100 hp.  The Electrically Controlled Variable Transmission is able to keep the engine near its peak power much better than a conventional step gearbox and the resulting performance is able to meet expectations for this kind of car.

A side benefit of the smaller engine is reduced weight, which offsets the additional weight of motor and battery.  The Prius uses an aluminum alloy engine block and other weight reducing techniques to keep the overall vehicle weight in line with similar conventional cars.

Atkinson/Miller Cycle

There is a practical limit to how much a car engine can be downsized.  The battery and motor that replaces the lost power tends to be heavier than the reduction in engine weight obtained.  The car must still be able to climb long hills found on public freeways at a reasonable speed, even after battery power is exhausted.  Toyota have chosen to give the Prius a 70 hp engine rather than reduce its size further and increase the electric motor and battery size.  This still puts the peak efficiency power above the most common power demands for non-aggressive driving.  The main culprit is pumping loss, that is the limiting of the power generated by a conventional Otto cycle engine by restricting the flow of air into the inlet manifold.  So, the Prius engine does not use the Otto cycle, but rather the "Atkinson" cycle.

In an Otto cycle engine, the fuel/air "charge" is drawn into the cylinder during the downstroke and then locked in by the intake valve closing near "bottom-dead-center".  The whole cylinder's volume of charge is then compressed and burned during the combustion or power stroke.  By contrast, the Atkinson cycle does not close the intake valve at bottom-dead-center but leaves it open while the piston begins to rise.  Some of the charge is pushed back out into the intake manifold (to be used in another cylinder).  The point at which the inlet valve closes is variable.  The volume of charge that is compressed and burned can thus be reduced without severely restricting the throttle opening.  This way of lowering power output without large pumping losses makes the Prius engine much more efficient than a conventional engine during most driving situations.  Argonne Labs measured the efficiency of the Japanese Prius engine to be 34% (good for any engine at its peak) at only 13.5 hp.

I mentioned above that the Prius engine is based on the same configuration and cylinder dimensions as the Echo engine.  It should be clear after this discussion of the Atkinson cycle that this similarity is misleading.  Under most conditions, the Prius will give better fuel economy than the Echo, even though the engine is pulling a larger and heavier car.  The improvements in economy are not only due to the hybrid system but also due to engine design.  Even when the ICE is running all the time, as in freeway driving, the Prius can give greater economy than the Echo.

At the end of this topic, I include some historical notes on the Atkinson cycle with links to more information on modern Atkinson and Miller cycle engines.

Offset Crankshaft and other Friction Reductions

When the reduction of friction loss is deemed worthwhile, relatively simple design changes make it possible.  We used the rubbing of the piston against the cylinder walls as an example of friction loss above.  This particular source of loss is most severe during the combustion stroke when the hot gasses push down on the piston.  The connecting rod that links the piston to the crankshaft makes an angle with the axis of the piston so that as the piston pushes down it is itself pushed to one side, hard against the cylinder wall.  By simply moving the crankshaft a bit to one side, the connecting rod can be made more upright during the combustion stroke so that the force of the piston against the cylinder wall is smaller and hence friction is reduced.  Both the Prius and Insight engines use offset crankshafts.  This is not limited to hybrid vehicles, since there is no loss of engine power, and the Echo also uses this technique to improve efficiency.

The Prius engine is also limited by the computer to a lower than normal maximum spin rate of 4500 r.p.m.  This reduces the peak stress on engine components, making it possible to make them smaller and lighter, resulting in less friction loss.

Elimination of Enrichment

Since high power demand is met in the Prius by adding the electric motor and battery power to the engine output, there is no need to used an enriched fuel/air mixture to eke out every last bit of engine torque.  The elimination of enrichment does reduce the maximum available torque, but efficiency is maintained right up to that maximum instead of falling off above 80% as in conventional engines.

Cylinder Disabling

In an engine with a large number of cylinders, typically more than four, it is feasible to disable some cylinders for low power operation.  This is typically used on large, powerful engines so that fuel economy is merely poor and not truly awful.  The Prius, having a small, four-cylinder engine, does not use cylinder disabling.  Alternatively, it could be said to disable all four cylinders when it is running under electric power!

Historical Notes on the Atkinson Cycle

After some research on the Web to make sure that what I'm saying is correct, I'm confused as why the Prius engine is referred to as Atkinson Cycle and not Miller Cycle.  My tentative conclusion is that this differentiates it from similar engines that use a supercharger and deliver power comparable to or greater than a similar capacity Otto cycle engine, such as the Mazda Eunos 800M.  Here are a few notes from my research.

Nikolaus A. Otto

Otto was a German engineer who worked for many years to develop an efficient commercial internal combustion engine.  Other engines at the time lacked compression of the fuel/air charge and were appallingly inefficient, having little advantage over the steam engine.  He achieved this goal with the first four-stroke engine with compression in 1876.  Some sources give a "Langen" some share of the credit.  The high efficiency and power/weight ratio (for that time) made the Otto cycle engine an immediate commercial success.  Unlike today's Otto cycle engine's, the originals used slide valves rather than mushroom or button valves and a flame tube ignition system rather than spark ignition.

James Atkinson

Atkinson was a British engineer working at around the same time as Otto.  In 1886, he developed an engine that was about 10% more efficient than Otto's.  It is not clear to me whether this was his main objective or whether he came up with something different to get around Otto's comprehensive patents.  In his engine, the intake and compression strokes were shorter than the expansion and exhaust strokes.  This was possible because of a peculiar crank and linkage mechanism between the piston and flywheel, quite unlike the simple connecting rod and crankshaft of today's engines.  It had the additional advantage that the flywheel rotated once for every two pulses of the piston so that the valves could be operated directly from the flywheel shaft and a separate camshaft was not needed.  Unfortunately, the complexity of the piston linkage offset the simpler valve operation and the additional friction losses it created offset the efficiency gain.  As a result, Atkinson's engine did not compete with Otto's.  For more information about Atkinson's engine including a very impressive animation, see Matt Keveney's Web site.

Ralph Miller

Miller was an American engineer who in 1947 obtained the efficiency improvement of Atkinson's engine with the simpler piston linkage of Otto's engine.  Instead of making the compression stroke mechanically shorter than the expansion stroke, he arranged for the intake valve to stay open during the first part of the compression stroke.  Although the intake stroke brought the full cylinder volume of fuel/air charge into the cylinder, part of it was then expelled through the open intake valve when the piston began to rise.  The compression stroke proper begins when the intake valve closes and the charge becomes trapped in the cylinder.  The engine is sometimes referred to as a "five stroke" by thinking of the compression stroke as divided into the partial charge expulsion stroke and the true compression stroke.  Personally, I don't find this particularly useful since the piston still makes the same four movements per cycle.

The efficiency gain of the Miller cycle is offset by a loss of peak output power for a given engine size.  Since a larger engine would be needed to meet performance expectations, the efficiency gain is also somewhat offset due to the partial power problem.  However, the Miller cycle has found application when combined with a supercharger, which restores high power without damaging the efficiency.  An example is the Mazda Eunos 800M engine.

Another Modern Atkinson Cycle Engine

No research can be considered exhaustive unless it turns up something that contradicts the final conclusion.  Here, for example, is an engine called the Multi-Cycle Engine or MC-5 that uses a supercharger and still calls itself "Atkinson cycle".  The piston is guided by rollers to travel vertically (therefore not needing a skirt).  It operates one end of a short lever, to the middle of which is linked the connecting rod that converts the reciprocating motion of the piston into the rotation of the crankshaft.  The other end of the lever can be raised and lowered to adjust the upper limit of piston travel, thus varying the compression and expansion ratios.

Last edited October 21, 2001.  All material Copyright  2001 Graham Davies.  No liability accepted.