40%-Efficient Engine, Really?

Simple calculation proves it is true.

1 gal of gas equivalent electricity (33.6kWh) drives Prime for 133 miles. One gallon of gas drives it for 54 miles, 54/133=40.6%

 

It's actually a major misconception with hybrid cars that the main thing that improves the efficiency is the addition of the electrical drive system. It's not. The main thing that improves the efficiency of hybrid cars is the implementation of the Atkinson cycle in the engine instead of Otto. This means the engine holds the intake valve open for part of the compression stroke, allowing some air/fuel mixture to come out, reducing compression ratio. A Prius engine operates with a typical ~8:1 compression ratio, but ~13:1 expansion ratio. This allows the engine to extract more useful work from each combustion.

The disadvantages of the Atkinson cycle are significantly reduced power and torque, especially at low RPM. The electric system compensates for this by providing high power and torque (which it's perfectly suited for) while the engine is at low RPM.

This is why when you look at two cars, one hybrid, one not, you can see increases in the highway fuel economy on the hybrid car (where an electrical system theoretically has almost no advantages). Regenerative braking obviously greatly improves fuel economy in city driving.

Another note here is that some of the more modern engines that can use an Atkinson cycle via VVT still won't be able to achieve the same efficiency of a dedicated Atkinson engine (without special fully variable compression and expansion ratio engines - Mazda's Skyactive engine is the only one I'm aware of currently). On the more modern Toyota engines, they have ~10.5:1 compression and expansion ratio. When using VVT, they can't change the expansion ratio, only the compression ratio, so with an expansion ratio 10.5:1 versus a hybrid's 13:1, they are not able to operate as efficiently.

Hope you liked this technical post!

 

It is a good start:

Let me offer some suggestions:

• Prius hybrids blend electric, Atkinson engine, and control laws to achieve superior efficiency.
• Below the hybrid critical speed, ~42 mph:
  • The Prius switches between engine OFF/ON and maintains speed in electric mode during engine OFF states.
  • The engine runs at higher, more efficient power levels, to bank the excess power as battery charge, target 60% SOC.
• Above the hybrid critical speed:
  • Engine runs all the time with high power demand like climbing a hill comes from the electric motors
  • Descending hills the car banks the excess regeneration into the traction battery

Your description of the Atkinson cycle is good. My suggestion is the control laws blend the ICE and traction battery to keep the Prius engine at the sweet spot of peak efficiency. The battery power either adds enough to keep the engine in the sweet spot or draws power at peak efficiency to bank for later use.

Bob Wilson

 

My understanding is a Miller cycle is just an Atkinson cycle with a turbo or super charger. It does have to be done carefully as boosting a low compression stroke ratio can result in needing a higher octane fuel to avoid detonation.

In theory, a Miller cycle could support a 6-to-1 or even lower compression stroke ratio and with the supercharger run up to higher power levels without having to open the throttle plate. One of significant energy losses are running with the throttle plate partially closed. So the Miller cycle allows a larger range of wide-open throttle operation with the supercharger adding the additional power ... carefully.

Bob Wilson

 

Agreed, my post above does not get into engine efficiency optimization via the ECU choosing when to use electric power versus engine power or using the CVT/HSD to control engine/electric motor speeds, which obviously can help keep the engine in the "efficiency sweet spot." My point is that the fundamental reason for greater fuel economy of the majority of hybrid vehicles is not the addition of the electric drivetrain, but the greater efficiency of the Atkinson cycle.

 

There is a fantastic in-depth article comparing the Atkinson and Otto engines and the associated thermodynamic calculations.

https://brandonyangcad.weebly.com/uploads/1/8/3/2/18324641/atkinson_cycle_report.pdf

The math is pretty deep, but the diagrams and the conclusions are fairly understandable.

 

Seriously getting my engine geek on in this thread. I also love this concept, though I doubt your average consumer would ever want a car with two gas tanks and have to fill up each every visit to the gas station:

 

Although I've seen different definitions.

The "true" Atkinson cycle uses a linkage to mechanically change the length of the intake/compression strokes relative to the power/exhaust strokes. AFAIK, the only true Atkinson cycle engine in production today is the Honda EXlink engine used in some Japanese home generator applications.

The original Miller engine used a variably actuated valve for bleeding off compression. Some consider the supercharger (whether belt or exhaust-driven) to be the important feature of the Miller engine (which is why most automakers use "Atkinson cycle" to describe late intake valve closing), some consider the LIVC nature to be the important feature (which is why Mazda calls their LIVC engines "Miller cycle" regardless of whether they're supercharged).

Although, the engine is most efficient somewhere in the ballpark of 2000ish RPM, and as all energy in the car ultimately comes from the engine (edit: I didn't catch that I was in the Prime subforum, so not all of the energy comes from the engine, but I was replying to a comment about a traditional Prius, where it does - and if you're forced to operate in charge sustaining (HV) mode, it still holds true on a Prime), if you're running the engine at 2000 RPM and the high voltage current coming from the battery is either zero or positive, the powertrain is operating efficiently. If you're in urban or suburban traffic, it's better IME to accelerate on the ICE and then cruise on electric, than it is to burn up electricity accelerating, and then have to fire the ICE at light load to recharge the battery.

I always find it hilarious when my Prius shames me for using too much accelerator, when I get ludicrously good mileage scores for that trip - a couple weekends ago the thing actually shamed me for getting 84.1 claimed MPG:

 

Yep

I also get flunking grades during some of my best fuel economy runs ;-)

 

All great points in this thread...

1- Roy2001 on the energy calculation proving the 40% engine efficiency on the Gen 4,
2- Kevin_Denver on the benefit of the Atkinson Cycle, and bwilson4web, Oniki, and bhtooefr on the ability of the Prius to keep the engine in its sweet spot on the highway, and
3 - the driving style of bhtooefr (the same style I use) to keep the engine (in HV mode on our Prime, or anytime on our Gen 3) in its sweet spot at lower speeds.

I found the quote in my signature (from ItsNotAboutThe Money another PriusChatter) summarizes all three points very nicely...

"The motors allow the engine to be at efficient loads on an Atkinson cycle.
It's the secret of the Hybrid Grail: the engine and the motor are one."
-ItsNotAboutTheMoney

 

Velomobile, Cycling & Walking seems to the most enery efficient.

 

{Chart snipped}
Seems a bit odd that a chart including Tesla and Leaf, represents commercial airliners by choosing a Boeing 727, which went out of production in 1984, and was retired from all U.S. fleets by 2003 due in large part to its expensive taste for fuel.

The only airline still flying them -- just three units -- is in Iran. And probably only because of economic sanctions making it difficult to upgrade to modern models.