Atkinson-cycle engine in Insight?

[rathacat]

Has anyone thought of trying to use an Atkinson-cycle engine in an Insight?
The newer Toyota Priuses get a lot of their efficiency from their 1.5 L Atkinson-cycle powerplant.

I don't know if you could stuff a Prius engine in an Insight, but one might be able to rebuild an Insight block into an Atkinson-type. Would low power at low revs be a problem with the Insight's gearing.

I just thought that the efficiency of the Prius engine combined with the light weight and aero design of the Insight might make one heckuva gas hybrid. Or is the incompatibility too great?


"Hurricane, 2001 Manual Transmission Insight, 69.3 mpg lifetime at 169,000 mi, battery replacement by Honda early 2007 for $1500 (Thanks to Honda USA for making the local dealer back down from the original $3,000 price. I was willing to pay something, since Hurricane was over the 150,000 mile mark.) Lives in the mountains and crosses a creek whenever I take her to town. Named because she was in Southeast Texas during Hurricanes Rita and Katrina. Tough, gutsy little car. Love her!"

[clett]

Swtiching to Atkinson cycle would make a huge difference to the economy of the Insight.

When Toyota engineers sat down with the brief to double fuel-efficiency for the Prius, they had to think of alternatives to existing engine technology (Otto cycle).

The amount of energy you can get out of a litre of fuel is proportional to the expansion ratio of the engine. That means, the longer the piston travels on the expansion stroke, the colder the exhaust gases and therefore more work has been taken out of the heat from combustion to drive the car. It makes for great efficiency.

This is mainly why diesel engines are so efficient.

For example, the thermal efficiencies of various engines are closely linked to their expansion ratio:

20:1 - 42% efficient (VW TDi diesel engine)
13.5:1 - 35% efficient (Prius engine)
10:1 - 28% efficient (good normal Otto cycle gasoline engine)

Toyota engineers therefore wanted to use the Atkinson cycle (actually Miller cycle, which is a little different) to be able to use a more efficient engine, and also use it in its most efficient map most of the time.

However, the problem with using these engines is that they have serious problems with lack of low-rev torque and responsiveness, so you can't hook them up to the wheels directly without some form of assistance.

This is why the Prius ended up as a hybrid, the battery-motor makes up for the inadequate driving properties of the Atkinson cycle engine. A lot of people think the benefit of the battery is regenerative braking, but this is not why they did it.

Honda probably would like to have used an Atkinson cycle engine in their hybrids, but it would be very difficult to get around Toyota's HSD patents. Their way out of this, of course, is to go series-hybrid.

[flunkysama]

One disadvantage to employing the Miller/Atkinson cycle is that exhaust temp is lower and so converter light off occurs later. Honda probably felt they could not keep the ULEV/SULEV rating and use the Miller/Atkinson cycle.

Also, IMHO, honda seems to avoid licensing others patents, and there is a patent on the Miller cycle.

[jat1793]

Expansion ratio?

Is that the ratio of air to fuel?

[shifty35]

Originally Posted by jat1793

Expansion ratio?

Is that the ratio of air to fuel?

Expansion ratio is the ratio of the volume of the compressed air / fuel mixture at the beginning of the power stoke to the volume at the end of the power stroke.

For traditional Otto Cycle engines, the compression ratio is the same as the expansion ratio.

The Insight has a compresion and expansion ratio of 10.8:1 on the 5 spd and 10.3:1 for the CVT compared to the expansion ratio of 13:1 for the Prius. Due to the Atkinson cycle, the Prius will have a lower compression ratio to prevent pinging on normal gas. This way it can extract more mechanical power per combustion cycle, while using the same amount of fuel as a smaller displacement engine. The extra expansion ratio will also drop the temperature of the exhaust gas as previously mentioned - the extra energy in the power stroke has to come from somewhere.

[kapps]

For those of you who don't know what an Atkinson or Miller cycle engine is, they both leave their intake valves open for part of the compression stroke. As shifty said, on the Atkinson cycle, this allows air to flow out, reducing the compression ratio but allowing a full expansion ratio. The Miller cycle is the same but uses a supercharger to compress against the piston for that initial bit of the compression stroke. This forces more air into the cylinder allowing for more power.

[clett]

Actually, a real Atkinson cycle is like this:



The Miller cycle is just a "simulated" version of Atkinson cycle that can use a simple single-crank Otto engine as the basis (by closing the intake valves late as mentioned above).

[shifty35]

Heh, you're close Clett... if you read farther down that page, there is an explanation. True Atkinson cycle allows for all 4 stokes to happen on one turn of the main crankshaft (vs. 2 in an Otto engine), but he failed to include the different length intake / power strokes in the animation.

I happened upon that a couple days ago while researching.

[rathacat]

It sounds as though the main obstacle in building a Miller/Atkinson cycle Insight is Toyota's patents.

If one wanted to do a one-off car, one could use the existing engine and just change the intake valve duration, probably by changing engine control firmware. I wonder if the size of the electric motor would have to be increased to provide more boost at low revs.

I suppose that Toyota could build an Insight clone using their technology (thus avoiding the patent problem). Honda might not object, since they don't seem to be using the aluminum body and panned rear wheels that enhanced the Insight's aero performance. All the present Honda hybrids are steel-bodied cars, as far as I know.

If anyone has an Insight with a dead engine, I might be interested in buying it for experimental purposes.

I don't want to use my Hurricane, since she works so well as she is.

[shifty35]

Since the valve timing is mechanical in the Insight, the only way to make the intake valves open later is to regrind the cam. An adjustable type cam gear could work but would also change the phase of the exhaust timing, since it's just a single cam.

The other problematic effect would be reduction of the net displacement of the engine. I'm not sure how well the Insight would drive with even less displacement.