One common topic on this forum is type of gasoline engine powering a given hybrid. For those of you that keep reading "Otto" and "Atkinson" but don't really know what that means beyond a name (myself included), these animations are pretty interesting:

http://www.keveney.com/Engines.html

Each one has a big overall animation and then a step by step walking you through exactly how it works. Very cool.

 

Those animations are cool... but its important to note that the atkinson engines in our vehicles are atkinson "CYCLE" engines. While the atkinson engine has a different compression and exhaust stroke because of the extra linkages, an atkinson CYCLE engine accomplishes the that feat by changing the valve timing. I THINK the valves are left open during part of the compression stroke. The internal guts on the otto and atkinson cycle engine are nearly identical.

 

Ah thanks for clarifying, I was wondering why the Atkinson engine there looked like that - he calls it an Atkinson Cycle Engine later in the text too. I assumed it was metaphorical.

So are you saying that the Prius engine really just changes the valve timings to get one revolution to cover all 4 steps out of an Otto engine? I guess I still don't understand what it is about the Prius engine that:

1) Makes it start and stop so cheaply

2) Requires it to warm up, making you basically drive on electric while it idles at very low RPM in cool weather.

 

I wish I could claim to be an expert on the mechanics of the thing.... but I'm not unfortunately. I know for sure the extra linkages aren't there. Fro example as I understand it my FEH has a 2.3L I-4 engine that looks substantially similar to the normal 2.3L I-4, however the valves are timed very differently. The result is an engine that gets much better gas mileage, but has poor low end torque (hence acceleration would be dismal). However by tacking on an electric motor (which have very good low end torque) that particular disadvantage can be largely eliminated. However, If I got too far into describing whats going on, I'd be guessing.

 

MG1 & MG2 are the two electric motors/generators. One of them starts the internal combustion engine. A key to Prius operation is that the electric motors turn instantly into generators when the computer tells them to, for regenerative braking. The car keeps borrowing energy from the batttery pack and then putting that energy back in, on a continuing basis.

I'd say you have this statement backwards. The computer is looking for a certain minimum temperature on the internal combustion engine, and has the car drive under power of the internal combustion engine until fully warmed up. It may use some electric power, but won't run solely on electric until the internal combustion engine is fully warmed up, and won't shut that engine off until it is fully warmed up.

Harry

 

Really? I've noticed that after the car is started for the first couple minutes in cool weather ("S1" I guess), the battery is actually being run down for any significant accelleration, and the gas engine doesn't rev up. It just sits at one constant idle regardless of whether you're going 1mph or 30mph. Then when it switches stages if I'm driving say 30mph I can audibly hear the gas engine kick into a normal RPM range.

 

The Prius version of the Atkinson cycle leaves the intake valves open for nearly half of the compression stroke. This actually exhausts some of the air-fuel mixture back into the manifold but not so far that it escapes. The effect is the compression ratio is only 8-to-1.

The valve eventually closes and the trapped fuel-air mixture is compressed to about 8 bar or ~104 psi when it is ignited by the spark plugs near top dead center. Then the heated gas expands nearly 13-to-1 before the exhaust valves open and the power stroke ends. So it is 8-to-1 to compress but 13-to-1 to expand.

So turning over the engine is very easy because it doesn't have to work so hard compressing the fuel-air. Also, less power is consumed on the compression stroke so more power is delivered to the shaft.

Now the downside is the power of an engine is a function of how much fuel-air passes through the engine. The 1500 cc Prius engine has less fuel-air so the maximum power is closer to 70 hp. But the exact same 1500 cc engine in my wife's Echo can generate 108 hp.

Does that help?

Bob Wilson

 

Correct but this appears to be a function of the early warm-up. Although an earlier model, I can significantly improve my daily commuting fuel by avoiding significant loads until the ICE has a chance to reach 40C or higher.

Your NHW20 had a hot-coolant bottle and this shortens the warm-up but it doesn't eliminate it. So if you can take 'the long way' out on the first drive of the morning, you can avoid having the vehicle try to use EV mode to get up to speed.

Although aluminum conducts heat well (some say too well in the winter,) it still takes a minute or so for the ICE to reach the 40C minimum and everything reach 'full power' temperatures. It makes sense to give the engine as much time as is practical to get warm before putting a serious load on it.

You still get better mileage at least starting on your journey but 25 mph is a reasonable speed limit for the first 2-3 minutes. Then you should be good to go.

Bob Wilson

ps. I have some data here:

http://hiwaay.net/~bzwilson/prius/pri_temp.html

 

It is "S1a" and was explained at Yahoo_Tech_group...
http://autos.groups.yahoo.com/group/.../message/18695

Ken@Japan

 

Thanks both, as always your info rocks.

That makes a lot of sense, Ken on what the Prius engine is up to in that first minute - they're saying the valves open before the gas even finishes burning, rapidly heating the catalytic converter up as though it were over a gas-lit furnace. That explains why you seem to be driving an electric car during that time... you are. The gas engine can't get much done if the valves are opening before they do much work. Very smart on Toyota's part, as always.

That Prius Group sure is hard to access! It's too bad the group posts are publicly visible - had to login to my never-used Yahoo account and all just to read, kinda weird.

So that's interesting about the "Atkinson Cycle" Otto engine - so basically because it does a little less compression and a little more expansion it wants to basically turn over at a relatively constant rate.

So my next question is that on the MFD I've noticed that if I press down on the gas even a little, the electric motor seems to kick in briefly then kick back out - is it being used to increase the rotation of the ICE? Does the ICE basically turn very efficiently at one constant RPM but takes the electric motors slowing it or accellerating it to vary its RPM in either direction?

Has anyone ever tried a catalytic converter heater instead of an engine block heater? Seems like heating the cat up before driving would have a bigger impact on the first few minutes' efficiency than heating the engine block... smaller part to heat up too, and maybe more open to access.