Hi folks,

I was asked how hybrids work on the highway and thought it best to share what I've learned with the community. I'm skilled with the Prius, the 2001-03 model, and some understanding of the Honda IMA systems. For this discussion, I'll offer two Toyota cars we own because they have the same engine block with 1500 cc displacement:

2001 Echo, 2105 lbs, 32/38 MPG, 108 hp, automatic
2003 Prius, 2765 lbs, 52/45 MPG, 70 hp, automatic

MPG - city/highway when sold

The 2001 Echo measured 38 MPG versus the same sized engine Prius getting 45 MPG so the effect is real. One clue comes from the lower power output of the Prius engine.

The Prius engine uses an Atkinson cycle that reduces the compression stroke to 8:1 while the expansion ratio is 13:1. This means the hybrid engine has lower compression losses and converts gas to power more efficiently than the Otto cycle, Echo. The Honda Civic Hybrid also tunes the engine to be more efficient than the gas only version because the Integrate Motor Assist unit provides the low-end torque needed to perform efficiently.

SCORE: hybrid ICE is more efficient than Otto ICE.

Correct! Both the Toyota and Honda systems use the battery and powerful motors to optimize the engine performance.

This is incorrect. All hybrids, even GM's hybrid lite models, stop their engines when not moving at red lights, stop signs or traffic. They all restart their engines within a fraction of second when the driver accelerates. During this initial acceleration, the battery is supplying the power to accelerate. In contrast, the gas only cars are burning fuel at idle and handling all of the acceleration energy.

Except that isn't what the EPA found: Echo 38 MPG and Prius 45 MPG. What also happens is the Prius employs a kind of 'overdrive' mode in the transmission. It reverses the power flow to take some power from the larger motor generator connected to the wheels to drive the engine slower for more torque and higher efficiency. This power actually flows from the smaller motor generator gear back to the larger motor generator, a type of energy recirculation.

Now it turns out that the Prius has a 'sweet spot' at 68 miles per hour. This is the fastest speed that gives excellent highway performance, ~52 MPG. At higher speeds, my model Prius has a rapid fall-off in performance that was fixed in the next model year, the 2004-current Prius.

It is inaccurate to claim "5% of its power" because the hybrid engine has a smaller power output, 70 hp versus 108 hp. The vehicle drag may be identical but the hybrid engine has much better efficiency when converting gasoline energy to mechanical power.

Actually the engine displacements and blocks are identical between the Echo and Prius. It is the Atkinson Prius cycle versus the less efficient Otto cycle that gives better mileage.

A little confusing but I think you are suggesting putting the Atkinson cycle engine in the Echo. Yes, it would be slower, much like the poor performance of the 1980s economy boxes. But the electric motor and battery solves the problem of slow performance of the vastly more efficient but less powerful ICE.

Bob Wilson

 

Excellent, Bob!
I would like to add to the above comments, representing the Civic Hybrid point-of-view.
In the Honda Civic Hybrid, the ICE IS smaller than maybe a comparable vehicle (say the regular Civic). That, and its advanced engine design allows the MPG to be higher.
And, Bob, your last comment about the slow performance issue resolution is one of the main reasons for the electric motor and battery system.

Also, gathering "free" regenerative braking somewhat diminishes the losses from the conversion of one form of energy to another. This is because not ALL of the juice put into the hybrid battery comes directly at the expense of the ICE. While slowing down, or stopping, much juice is put into the hybrid battery. These are times when you don't mind the regenerative braking at all. In fact, used properly, this greatly increases the life of your brakes (and braking components).

 

The question the person wanted answered would be... if you removed all the hybrid stuff, Batteries, motors, etc, and just ran the same car with only the atkinson engine, his theory is at highway driving (relatively constant speed), the atkinson by itself would get better gas miliage than the hybrid system. Once up to the speed. My take is that because of the constant infusion of the electric motors power and the lower RPM's that the atkinson is able to run at in the hybrid system, The Hybrid system would still get better gas mileage at highway speeds. ( even forgetting the rampo up to the speed)Even at 70 miles per hour, there are times on very slight downgrade that our ICE shuts off, and we essentially are running on the electric motors/slight coasting, and at the same time the batteries are also getting recharged,the display flips around crazily between adding electric power and recharging. Yet we keep the same 65 or 70 miles per hour.

 

Hi,

This becomes a bit difficult because of the contradictions. There are experiments that could be performed to support either point of view. However, in real life, you are correct. We just don't find steady state conditions on the road.

If we were to do the following:

• run the car on a dynometer so there are no variations
• weld-up a fixed gear between the engine and wheels
• start the car by running the dynometer and letting it warm-up

In this case, the fixed ratio gear vehicle has on the dynometer an advantage. However, it would be nearly unusable in real-life. Any trip would require a 'pusher' car to get the fixed gear car up to speed and start the engine. Thereafter, the car would have a narrow range of speeds and any significant grade would stall it out and require the 'pusher.'

The closest thing to a fixed gear ratio is a manual transmission vehicle. One of the favorite 'tricks' of the anti-hybrid, car authors is to compare a manual transmission diesel to the automatic transmission hybrid on the highway. But when you look at the automatic transmission version of the diesel either on the highway or in the city, the mileage is dirt.

There is a Toyota SAE paper that maps hybrid performance to other vehicles and ONLY a handful of manual transmission vehicles compare and only on the highway. In all other driving modes, the hybrid excels.

In the real world, you are absolutely right. So I would point out that your driving is in "the real world" and you don't have the luxury of using a manual transmission vehicle at one speed on the highway. Wish them luck if that is all they drive.

Bob Wilson

 

BrisPrius,
I think you are correct in interpreting the question. I also think your assertion that the hybrid version would get slightly better MPG is correct, also. Even though the hybrid components add weight. It would probably be pretty close - on the highway. In the city, there's no comparison. I think the hybrid wins the MPG battle, hands down.

Bob, your point about "real world" is spot-on, too. In a lab-created environment, we can probably concoct a way to make the normal ICE version get higher MPG than the hybrid version with the same ICE. But would the vehicle be NEARLY as drivable as these hybrids are overall? I think not. That ICE only version would be REALLY slow off the line all the time; it would be quite slow in passing/acceleration situations, climbing hills, etc. This is why hybrids are a reasonable method for attaining higher MPG - they have very few negative issues relating to their driveability and performance.

 

If you built a HF model (I believe that was Honda's old designation for high FE version) of the Civic with the ICE from the hybrid - it would be pretty close to the HCHII as far as highway mileage. It would definitely be as good as or better on a flat highway. And .... it would be a lot cheaper. It would be pretty slow but the main annoying thing would be having to rev the engine to get decent acceleration. I remember being impressed that the HCHII does not have to rev the engine much in normal acceleration for a 1.1 liter engine - and its revs at highway speeds are very reasonable. But put it in cruise control and on the hills, you will get some idea of what a HF model would be like - very annoying IMO.

I think the HF model (or equivalent) will come back as it makes sense since it is much cheaper.

 

I think it's a 1.3 liter engine. But you are probably right about HF-like cars making a comeback. The killer for today's HF-like car is trying to reduce the weight without resorting to exotic (and expensive) materials. Today's average small car weighs A LOT more than small cars of the 70s and 80s.
Now the Lotus Elise is pretty light... Imagine an engine in that thing designed for high MPG! It would STILL probably be pretty quick, just because it IS so light. Sign me up!

 

Yep, I think a 1.1-1.3L very lightweight vehicle might get both good FE and good accelleration... Oh yeah, they are called motorcycles