MPG with ICE Only

Zero. If you disconnect the motor-generators the car won't move, so you get zero mpg.

Tom

 

There are a number of things you can do to get close to an answer.
.
You can drive steady-state on the highway, which with the battery
at 60% SOC and not appreciably charging or discharging, the
electric motors only work to shunt about 28% of the engine's
output torque between themselves and then feed it right back
to the wheels to keep the car moving. The MPG vs speed tradeoff
hinges around 60 @ 60 in good weather, i.e. you go faster you
get slightly less MPG and above certain thresholds, you go slower
and get higher MPG.
.
You can try to drive without the hybrid-system advantages such
as regen braking and auto-stop. Judicious (mis)use of Neutral
can have you completely on the brakes for stopping, idling at
lights and at the drive-thru, and coming fairly close to the
conditions of a conventional car. The powerplant can still deliver
upwards of 50 MPG, depending on traffic conditions and such.
.
But you can't "unhook the electric motor" and drive around, since
the whole HSD / powertrain depends on the interplay of the TWO
electric motors. See, if you go an try to unhook one of them,
the other one will sneak around behind you with a baseball bat
so you'll never be able to do it. [Sorry, just caught up on the
Abstruse Goose archives...]
.
_H*

 

The basic response is "Why ask?" -- but more on that later.

It is really hard to make such a guess. Without the battery hooked up, you'd have a car carrying around an awful lot of extra weight (the electric motors and the battery), and that would really hurt mileage. Also, the gasoline engine (the ICE) is an Atkinson engine designed not for power but for efficiency. It would be heavily underpowered and would not produce high MPGs carrying a heavier car than such an engine is designed to pull. Further, you'd have to add a heavy transmission and all of the pumps would have to be changed to mechanical (pulley) power, unless a much larger 12V battery was added.

Over all, if you went through all of the redesigning, you'd have something akin to an overweight and underpowered Corolla or Civic.

Back to my original response: Why ask?

 

The engine won't even start without the electric motors. Even if you managed to start the engine, it'd be so sluggish and slow that you'd feel like a huge 16 wheeler.

But I guess that's not what your looking for either, the closest comparison would probably be the Toyota Yaris, which uses a VERY similar engine, except without the Atkinson cycle and a lighter and smaller body. The Yaris gets 29/36mpg.

 

I ran into a situation like this awhile ago...I can't remember how I got the battery drained so much. But the answer is: max, 36mpg. If you run A/C and various other things that drain the battery, and drive in stop-start-stop city traffic, the battery will run out quick, and the acceleration will take a nose dive; and the fuel economy will max out at 36mpg. So the electric assist averages about 15mpg or so.

 

you have to consider with that 36 mpg you are recharging the battery so 36 can't be an accurate amount. The closest you could come is at a stable speed, with 0 high volt battery amps but this varies and you could never get a normal driving average.

 

<opps>

 

The simple answer is battery charge has nothing to do with the mileage. The Prius battery is too small to provide more than a mile or two yet we drive our Prius 100,000s of miles without ever charging it. All of those miles came from the gasoline we burned in the engine.

2001-03 PRIUS

The battery is part of a sophisticated transmission. So we can compare two vehicles with the same engine, similar body and different transmissions using the USA EPA numbers:

• 30 MPG - 2001 Echo, 1.5L, automatic
• 41 MPG - 2001 Prius, 1.5L, electronic-automatic

So the simple answer is the electronic-automatic transmission was good for about 11 MPG, a ~33% improvement in mileage. The battery is just one part of that transmission.

2004-09 PRIUS

If you want to see more modern versions of 1.5L, automatic transmission vehicles:

• 31 MPG - 2009 Yaris, 1.5L, automatic
• 46 MPG - 2009 Prius, 1.5L, electronic-automatic

The mileage increase is 15 MPG, ~50% improvement. Again, same engine but an improved electronic-automatic transmission. But better performance is coming with the 2010 Prius.

2010- PRIUS

The 2010 Prius will use a 1.8L engine so we can make this comparison:

• 28 MPG - 2009 Matrix, 1.8L, automatic
• 50 MPG - 2010 Prius, 1.8L, automatic (projected)

Later this year we're looking at a 22 MPG increase, this will be a ~79% improvement for a 1.8L engine yet the battery is the same number and identical modules as in the 2009 Prius.

SUMMARY

The battery is just part of the transmission, a remarkable device, which lets the Prius run about using very little gas.

Bob Wilson

 

With the Atkinson cycle, the effective engine capacity is reduced. The intake valve is closed, on the 2G car, between 72 degrees and 105 degrees after bottom dead centre. If the stroke is sinusoidal - which it isn't, quite, I think - it would change the capacity from 1.5 litre to 1.0 litre maximum (72 degree closing) to about 550cc minimum (105 degree closing). I would anticipate maybe a 1.2 litre effective capacity on the 2010 Prius, but we don't know the VVTi parameters yet.

This is why the Atkinson cycle is described as having a poor power-to-weight ratio: it has the maximum power of a 1.0 litre but needs the engine block of a 1.5 litre, hence more metal, for the expansion ratio. I do wonder if better results could be achieved if the engine block were designed specifically and only for Atkinson operation - 1NZ-FXE and now 2ZR-FXE share large parts of their design with 1NZ-FE and 2ZR-FE. While the NZ engine was designed primarily for the Prius, I wouldn't be surprised if more -FE (Otto) engines have been built than -FXE.

Displacement is a fairly meaningless statistic on a regular Otto cycle engine, but almost completely meaningless on the Atkinson cycle.

It's my belief that most of the fuel economy benefit comes from Atkinson cycle, and is much less to do with the electric motors. The electric system allows a) a very efficient continuously-variable transmission and b) makes the Prius a practical car, helping out where the engine is weak (high torque at low revs). The battery is just used to smooth out the load, where possible keeping the engine in its efficient mid-speed mid-load range.

 

The original Atkinson Cycle engines used a complex double crankshaft linkage which allowed for a longer power stroke. The current crop of Atkinson Cycle engines effectively do the same thing by holding open the intake valve during part of the compression stroke. It's not quite the same thing, but the net result is mostly the same.

An interesting aside is that the complex linkage was the result of the patent on the Otto Cycle. Atkinson needed a different method because Otto had him locked out. Sometimes these annoying patent battles lead to good things.

Mostly true. Regenerative braking, while not the major contributor, is also significant.

Tom

 

Certainly Mike's description of the Atkinson cycle is accurate but we are left with deciding which displacement to use:

• compression stroke - ~800 cc
• expansion stroke - 1,500 cc

What the hybrid transmission, including the battery, does is allow the same 1.5L engine block to perform like the 108 hp, Otto version but sip gas more like a 1L engine. This would not be possible with ordinary transmissions.

Bob Wilson

 

Well put. You have condensed pages of technical discussion into two well formed sentences.

Tom

 

Bob is referring to the Classic Prius but for the improvements in the Iconic Prius, the peak hp is the about same as 1.5L Otto version (110hp) but it performs like a 2.0L Otto. 0-60mph in 10.5 secs while 30-50mph in 5 secs. There is no other car with Otto cycle that weights ~2,900 lbs with 110hp can pull those numbers.

 

Thanks!

You brought out a good point, 0-60 times vs. weight, as a measure of maximum acceleration, the power available to reach a given kinetic energy. I'll have to do some 'back of the envelope' notes and if I add it to my original posting, I'll certainly include attribution.

The only problem is 0-60 and vehicle weight as a measure of wheel power is not ordinarily associated with MPG, vehicle fuel efficiency. But if we go into some really snarky analysis of the EPA test protocol, it plays a part.

Bob Wilson