Gen4 Prius ICE Details: 40% Efficiency

Owch!

The end of April, I searched the USA based SAE site for Toyota papers and only found one minor paper. Usually Toyota has a set of papers on the new Prius discussing the technology. Now it might be the papers have not been indexed but I've also been aware there is a Japanese SAE and they just finished their Spring conference. Sad to say, I could not easily find the Toyota papers on the Prius at that site, yet.

If anyone has any luck finding the Toyota paper, I would appreciate a link.

Engines hold a special place in my interest and the cooled, recirculated exhaust is important but the article also references to swirl. I'm especially interested in the variable valve timing, angle and duration, because this holds the promise of further reductions in throttle-plate, pumping loss. However, there are often papers that discuss associated system control laws that improve total vehicle improvements. Often an article about a technology highlights the 'awareness' of the article author who may not share my understanding of ICE technology.

Thanks,
Bob Wilson

 

Toyota Exhibits High-thermal-efficiency 1.5L Engine(May 23, 2015)

 

A measurable upgrade to the Atkinson cycle motor design. which translates to at least 10% improvement in MPG - especially during the winter time. . this motor will be Toyota's first stand-alone Atkinson cycle motor design .. Toyota. plans to put this motor into non-hybrid cars in Japan... when this motor hits the asphalt it will be very interesting ..

 

Combustion Development to Achieve Engine Thermal Efficiency of 40% for Hybrid Vehicles

Takahashi, Nakata, Yoshihara, Ohta, Nishiura

This paper describes some concepts like high tumble that are able to expand the EGR limit and to realize the engine thermal efficiency of 40%.

 

Is it just me, or does a casual skim of the wording in the reports infer a 40% increase in efficiency? Say compared to an alternative where they say:

Next gen Prius engine's thermal efficiency raised 1.5%.

 

I still don't care for the SAE web site but eventually thanks to the first link, I found and purchased copies of:

• 2015-01-1254
  Combustion Development to Achieve Engine Thermal Efficiency of 40% for Hybrid Vehicles
• 2015-01-1101
  Development of New Continuously Variable Transmission for 2.0-Liter Class Vehicles
• 2015-01-1268
  The New Toyota 1.2-Liter ESTEC Turbocharged Direct Injection Gasoline Engine
• 2015-01-1652
  Thermal Flow Analysis of Hybrid Transaxle Surface Using Newly-Developed Heat Flux Measurement Method

Now to find time to read them. <GRINS>

Bob Wilson

 

The paper provides a lot of details lay reports (even mine) miss:

• high tumble means more, cooled EGR can be used reducing the exhaust temperature, a major energy loss
• higher tumble and EGR ratio means a hotter spark is needed
• combustion is faster so the spark needs more energy to avoid arc blow-out

When we got our 1.8L Prius, one of my earliest benchmarks looked at EGR ratio as a function of rpm. The peak was about 3,100-3,200 rpm which remains my peak engine rpm when climbing hills or accelerating. But one thing bothers me.

"Figure 1. Engine operation range for different drivetrains" suggests the improved efficiency shows up at ~7kW (~10 hp) and above. To exploit this higher efficiency range, minimize engine operation under 10 hp. This suggests the Prius needs a wider and deeper operation of the hybrid drive system in the 0-10 hp range with attendant stress on the traction battery.

This same figure also compares a "MT" and "CVT" transmission and it shows a significant advantage of an "MT" in the 12 kW ( 16 hp) range. At lower and higher power ratios they are nearly equal. So why report "MT" unless . . . there might be a manual transmission version?

It took me nearly three years to finally understand Toyota's CVT. But an MT could eliminate MG1 saving weight or lead to a stronger MG2. The 'clutch' and shifting mechanism of an MT could be awesome.

This is from just a screen-read. I've usually found a paper copy read gives even greater insights.

Bob Wilson

 

Min 10hp threshold has been true since Gen1 Prius with 1.5L. With, 1.8L, they had to do EGR to reduce pumping loss further to maintain that 10hp. With improvement in EGR, Gen4 may be a bit better.

Interesting comparison with the MT transmission. Perhaps, it was used in the graph to show how efficient the new eCVT.

 

Bob is now in hog heaven

.

 

This is the figure we're discussing:

• I suspect Consumer Reports 'testing' will go GA-GA over this. I've long suspected CR's MPG test involves significant power mode. I'm not sure if they floor it but would not be surprised.
• Regardless of transmission type, the "operating line" is going to be the real tell.
• This chart extended to maximum power would be very interesting.

Bob Wilson

 

Ok, so they are just comparing Manual, CVT (mechanical) and HV (eCVT).

At 10hp, ICE is about 35% efficient but the Fuel Consumption Ratio (system efficiency?) is about 17%. That means eCVT takes 50% loss?

 

The throttle is mostly taken care of on C and likely Gen3. For most ICE runs with manifold pressure close to atmospheric with plate open 30-70%

 

I wasn't aware that anyone at CR knew how to "floor it".

 

I am reading there is no DI. They say mixture flow, so it looks like port injection to me

 

any data on MG sizes? increased? kWh of the traction pack?
.

 

I'm not sure the data supports this. I just printed off the paper and can now do a 'proper read.'

Strange, I suspect younger folks who learned to read on the screen have higher reading rates and comprehension than us 'papyrus' trained, old farts. <GRINS>

Bob Wilson

 

I was just going by the two charts you posted. The top one said ~35% BSFC. Bottom one said ~7% (looking at it again, it is no way 17%) at the 10hp line.

 

I understood it as the percentage of power that makes it to the wheels.

In another word, transaxle efficiency. Perhaps, Bob can post more detail what it means.