Toyota Claims Bigger Engine are Better for Fuel Economy

[Daox]

The blog is more of a publicity tool and brings in a lot of people that wouldn't normally come to the forum and/or site. Its just a different way to help expand EM and get new users.

[bwilson4web]

For me, it wasn't the blog as much as reading the forum threads and projects. I've found folks who grab their tools, perform the experiment and post the results. Finding a community grounded in reality makes it easy (perhaps too easy?) to share my data and reference material.

In fairness, I'm a moderator at GreenHybrid.com, a hybrid advocacy group, participate in PriusChat.com, the technical forums and Prius specific advocacy, and "Prius Technical Stuff," a group like ecomodder specific to the Prius. But I'm not here to 'bang the Prius drum,' an advocate, as much as another 'gear head' interested in the technology across a wider base than just the Prius.

Bob Wilson

[roflwaffle]

Originally Posted by bwilson4web

Using your charts, it looks:

• Prius - BFSC less than 230 g/kw-h between 2,200-3,400 rpm.
• 6-banger - BSFC less than 240 g/kw-h is between 1,400-2,900 rpm.
• 6-banger narrow - BSFC at 237 g/kw-h, 1,800-2,400 rpm.

Did I misread the charts?

No, you didn't misread the charts. However it appears you didn't understand what I was talking about. Specifically, you stated that the Atkinson cycle results in a substantial reduction in throttling losses in low power regions, however when comparing it to older engines the change in BSFC compared to load, adjusted for displacement/age of course, is more or less identical. In other words, the Atkinson cycle didn't do a lick for low load throttling losses as far as I can see, adjusted for the difference in overall efficiency/displacement. Granted, Toyota has increased overall engine efficiency, just like they did when comparing the two sixes made about a decade apart in the BSFC map, but all the Atkinson cycle does is allow for a cheap way to reduce the effective displacement, aka a cheaper way of destroking the engine.

Originally Posted by bwilson4web

When you get a chance, I'd recommend getting a copy of SAE 2004-01-0064 for this quote, "As a result, the minimum specific fuel consumption of 225g/kWh has been achieved. . . " (pp. 7.) This paper is the source of the first graph and does an excellent job of showing the specific systems in the Prius. More importantly, it shows how the Continuously Variable Transmission keeps the engine at the best BSFC over a very wide, rpm range (the operating range line on that first chart.)

Send me a copy!

Originally Posted by bwilson4web

This operating line is the problem the old 6-banger could never solve with existing transmissions.

Well, lesse, the old transmission over the same speed range would see BSFC increase about 9% compared to the minimum, specifically a minimum of 237g/kWh and a maximum of 260g/kWh, and the Prius also exhibits a ~9% difference with a minimum of 230g/kWh and a maximum of 245g/kWh. The biggest difference is the more "open" ovals as engine speed increases, which is due to the offset crank and lower friction losses, not the greater expansion ratio AFAIK. Course, like everything either one of us, or possibly both of us, could be wrong, so in short we need more info IMO.

Originally Posted by bwilson4web

The Otto version of the same 1NZ-FE is in our Toyota Echo and the early Scion series. The same engine is in the Yaris.

A closer comparison would probably be the 2NZ-FE, although I haven't seen a BSFC map for it, and for that matter, since we don't know what the effective displacement of the 1NZ-FXE is, who knows what Otto cycle engine it would be equivalent to in terms of displacement. If we could slap something together with the same effective displacement, offset crank, all that jazz, less the greater expansion ratio of the 1NZ-FXE, then we could see what the difference is. Until then, all we can do is (fun IMO) internetz speculation.

Originally Posted by bwilson4web

But the mechanical aspects probably play another important part.

As the expansion ratio increases, the stress on the piston, rod, crank and cylinder head goes up right after ignition. It is equally likely that the 13-to-1 ratio seen with the 1NZ-FXE is a mechanical limitation. They didn't want to add the additional metal needed for a higher expansion ratio that might cause the engine to approach diesel weights.

Bob Wilson

I've never heard of a greater expansion ratio increasing stress outside of conventional engines. For those it's a given since the CR has to go up too, but if possible, it seems that having ER>CR would lower stress on the engine internals all things being equal.

All things being equal, an engine with a lower CR should see less stress on the internals, and by increasing the expansion ratio they're lowering EGTs/extracting more work over a greater time period, which should also decrease stress compared to extracting more work over the same time period. If anything I think the expansion ratio was dictated by what Toyota could do with the combustion chambers as opposed to the mechanical limits of the block.

[bwilson4web]

Originally Posted by roflwaffle

No, you didn't misread the charts. However it appears you didn't understand what I was talking about. Specifically, you stated that the Atkinson cycle results in a substantial reduction in throttling losses in low power regions, however when comparing it to older engines the change in BSFC compared to load, adjusted for displacement/age of course, is more or less identical. In other words, the Atkinson cycle didn't do a lick for low load throttling losses as far as I can see, adjusted for the difference in overall efficiency/displacement. Granted, Toyota has increased overall engine efficiency, just like they did when comparing the two sixes made about a decade apart in the BSFC map, but all the Atkinson cycle does is allow for a cheap way to reduce the effective displacement, aka a cheaper way of destroking the engine.

Then we'll have to agree to disagree. I can't ignore my data:



I was hoping the Autospeed article on EGR might have also given a clue.

In engines that use throttles, high pumping losses occur in part-throttle conditions. That is, the engine needs to do work in order to draw air past the partly closed throttle. One result of this is that as load decreases and the throttle is closed to a greater extent, Specific Fuel Consumption (ie the fuel consumed per power produced) becomes increasingly worse. . . .

Because half of the intake charge is pushed back into the manifold for the next cylinder taking in its charge, the throttle plate pumping losses have been effectively cut in half. Still, the results are quite telling on the road.

Originally Posted by roflwaffle

. . .
Well, lesse, the old transmission over the same speed range would see BSFC increase about 9% compared to the minimum, specifically a minimum of 237g/kWh and a maximum of 260g/kWh, and the Prius also exhibits a ~9% difference with a minimum of 230g/kWh and a maximum of 245g/kWh. The biggest difference is the more "open" ovals as engine speed increases, which is due to the offset crank and lower friction losses, not the greater expansion ratio AFAIK. Course, like everything either one of us, or possibly both of us, could be wrong, so in short we need more info IMO.
A closer comparison would probably be the 2NZ-FE, although I haven't seen a BSFC map for it, and for that matter, since we don't know what the effective displacement of the 1NZ-FXE is, who knows what Otto cycle engine it would be equivalent to in terms of displacement. If we could slap something together with the same effective displacement, offset crank, all that jazz, less the greater expansion ratio of the 1NZ-FXE, then we could see what the difference is. Until then, all we can do is (fun IMO) internetz speculation.

Just pointing out there are a bunch of 1NZ-FE engines around. Feel free to find a BSFC chart for them. However, the 2NZ-FE engines are an entirely different breed.

Originally Posted by roflwaffle

. . . I've never heard of a greater expansion ratio increasing stress outside of conventional engines. For those it's a given since the CR has to go up too, but if possible, it seems that having ER>CR would lower stress on the engine internals all things being equal.

The 1NZ-FE engine was more likely developed for the Otto cycle engines. The hybrid version is a simple modification of an existing Otto engine and that is probably what set the maximum expansion ratio. After all, it takes a pretty high octane fuel to run without knock at 13 to 1.

Originally Posted by roflwaffle

. . . All things being equal, an engine with a lower CR should see less stress on the internals, and by increasing the expansion ratio they're lowering EGTs/extracting more work over a greater time period, which should also decrease stress compared to extracting more work over the same time period. If anything I think the expansion ratio was dictated by what Toyota could do with the combustion chambers as opposed to the mechanical limits of the block.

Same answer, different path, no problem.

Bob Wilson