Background

Searching for a web page describing the Ford or Toyota Atkinson engine usually brings up an articulated crank-shaft engine. Although historically interesting it doesn't explain how an Atkinson cycle engine achieves efficiencies approaching modern diesel engines and makes it different from the traditional Otto engine.

Modern Atkinson Engine

The key technical aspects are: The following mpeg animation is described in the text below:

Start

Cyl 1 - Exhaust stroke beginning with exhaust valve open
Cyl 2 - Ignition of fuel-air charge, power stroke
Cyl 3 - Beginning in-stroke with intake valve open
Cyl 4 - Completion of intake stroke, compression stroke next

Power Stroke, 1st quadrant

Cyl 1 - Exhaust gas leaving cylinder
Cyl 2 - Power stroke early phase
Cyl 3 - Intake pulling charge from the cylinder 4
Cyl 4 - Part of intake charge is passing back through intake valve for cylinder 3

Because the intake valves of cylinder 3 and 4 are fully open, there is little to no restriction of the flow. In contrast, pumping losses are those that occur when the air has to pass the mostly closed, throttle plate. In an Otto engine, cylinder 3 would be sucking against the throttle plate.

Power Stroke, half-way

Cyl 1 - Exhaust gas leaving cylinder
Cyl 2 - Power stroke half way
Cyl 3 - Now drawing fuel-air through throttle, now some pumping loss
Cyl 4 - All valves closed as remaining fuel-air compresses

With half of the intake stroke donw, now cylinder 3 is seeing some throttle-plate, pumping losses as the intake air has to pass through the narrow opening of the throttle. But unlike an Otto engine, this happens only for last half of the intake stroke.

Power Stroke, 3d quadrant

Cyl 1 - Exahust gas leaving cylinder
Cyl 2 - Power stroke continues
Cyl 3 - Continues to draw in fuel-air mix by throttle plate
Cyl 4 - Compress of fuel-air mixture continues

Power Stroke ends

Cyl 1 - Exhaust gas done, intake valve open
Cyl 2 - Begin exhaust stroke, open exhaust valve
Cyl 3 - Bottom on intake stroke
Cyl 4 - Ignition of fuel charge

Intake stroke begins

Cyl 1 - Intake valve open and drawing fuel-air from cylinder 3
Cyl 2 - Exhaust stroke
Cyl 3 - Beginning compression stroke and pass charge to Cyl 1
Cyl 4 - Power stroke

Details

This cycle repeats for each cylinder, passing some of the charge from the first half of a compression stroke to another cylinder during the first half of the intake stroke of a second cylinder until the delayed-closing of the donor cylinder. But since the intake valve closing of the donor cylinder is variable, this also controls the amount of fuel-air charge in the donor cylinder. This provides another throttle plate function without the throttle plate restriction the source of 'pumping losses.' In fact, the next generation of variable valves eliminates the need for a throttle plate.

During the power stroke, a 13-to-1 ratio, the hot gas expands releasing the heat energy into mechanical energy. This expansion ratio is not as much as a diesel 20-to-1 ratio but it doesn't have to be. In contrast, the diesel that has to compress the air 20-to-1 while the Atkinson only has to compress it 8-to-1 or less. These lower pressures and forces reduce internal, mechanical friction. At idle, both the diesel and Atkinson engines have a minimum fuel burn to overcome internal friction from piston ring sealing and moving part friction. The diesel still has to sustain a 20-to-1 pressure ratio, which means a fixed engine overhead. The Atkinson engine has an 8-to-1 ratio (or less depending upon intake valve closing.) This significantly reduces the sealing ring friction. Finally, the hybrid electric vehicle just turns off the engine at idle but that is not all that happens at low power settings.

Fuel-to-Wheel Efficiency - the hybrid urban advantage

To simplify the math, assume that a 1.8L Atkinson engine starter and 2.0L diesel engine need the same 5 hp (3.7kW) starter requirement and this is equal to the idle engine overhead. So at idle, both engines need to generate 5 hp worth of power that is fully consumed by the idling engines. Now 5 hp is a special power case for an NHW11, 2003 Prius as this is also the power needed to cruise at 30 mph. For the diesel-only version of the NHW11, the engine has to generate 5 hp to over come internal friction and 5 hp to move the vehicle at 30 mph. Roughly half of the fuel burned is lost just keeping the engine running: The same would be true for the Atkinson engine except the hybrid electric drive plays a trick. It runs the engine at four times 5 hp, 20 hp: The fuel burn rate supports 20 hp but only 25% of the power is consumed turning over the engine. In contrast, the diesel is burning 50% of its fuel to keep the engine turning over. Once the battery is charged, the Prius turns off the engine and runs the car on the stored energy. This is the tank-to-wheel efficiency and shows up in the "City" EPA ratings: