Toyota Prius proves a gas guzzler in a race with the BMW 520

Chuck Branscomb · **Posted:** Wed Mar 19, 2008 12:51 pm

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Is this a case of man bites dog? A mid-size BMW sedan equipped with a diesel engine returned better fuel economy on a 545-mile, London-to-Geneva, run than a gasoline-powered Toyota Prius. A BMW 520d with a 2.0-liter diesel engine and regenerative braking (but no heavy duty hybrid features) got 41.9 mpg while the full-hybrid Prius, 500 pounds lighter, got 40.1 mpg.

Quote:

To find out we set a challenge: to drive a Prius to Geneva using motorways and town driving. The direct route is 460 miles but we drove almost 100 miles further to give the Prius the advantage of running in urban conditions where its petrol-electric drivetrain comes into its own.

We took along a conventionally powered car – a diesel BMW executive saloon – for comparison and drove both cars an identical number of miles (545).

http://www.technoride.com/2008/03/bmw_diesel_beats_prius_in_econ.php

Donnie Barnes · **Posted:** Wed Mar 19, 2008 1:03 pm

I'm too lazy to read the article, but can someone explain to me how a non-hybrid vehicle can have regenerative braking? Doesn't seem possible for the brakes to put fuel back in the tank, and with no electric propulsion mechanism...

Maybe the "brakes" engage a ratchet that winds a big spring and the ratchet is released when you accelerate?

--Donnie

Chuck Branscomb · **Posted:** Wed Mar 19, 2008 1:09 pm

It just uses it to "charge the battery and help power the electrical systems" as opposed to charging batteries for an electric motor to use.

shawnwhipple · **Joined:** Wed Aug 20, 2003 6:28 pm **Posts:** 603

I saw a experimental regenerative braking system on a bus once. The system worked by mounting a hydralic pump on the drive line. When you applied the brakes the pump engaged and forced fluid into a holding tank building pressure. Then when you accelerated the pressurized fluid was released back through the pump to turn the drive line to help the motor accelerate.

At the time I saw it on TV they said it was only useful in urban settings on heavier vehicles. Maybe BMW has improved the technology or maybe it was just a journalist that got their facts wrong? Don't laugh, it happens, even on the interweb. :lol:

Shawn

Donnie Barnes · **Posted:** Wed Mar 19, 2008 3:07 pm

What liquid would you use in the hydraulic system? Obviously not oil since while it will certainly pressurize, it doesn't compact (which is why it's generally used in the "normal" hydraulic systems we're used to). Or maybe the "tank" was really a big piston arrangement that actually compressed a big ass spring (which could be mechanical or could be compressing gas). Interesting idea, particularly with something like a bus that does pretty much nothing but stop and go in short cycles.

As for the regeneration charging the battery on a non-hybrid, I'd really like to see the math on *that* one. Don't normal alternators have significant drag even when the battery is basically fully charged? If so, I suppose you could add a clutch to disengage (like AC compressors have) and thus reduce the drag from the alternator when you didn't need it. *shrug* Still seems like yet another system that would take a LONG time to pay for the added complexity...

--Donnie

Carl Fisher · **Posted:** Wed Mar 19, 2008 3:34 pm

Donnie Barnes wrote:

What liquid would you use in the hydraulic system? Obviously not oil since while it will certainly pressurize, it doesn't compact (which is why it's generally used in the "normal" hydraulic systems we're used to). Or maybe the "tank" was really a big piston arrangement that actually compressed a big ass spring (which could be mechanical or could be compressing gas). Interesting idea, particularly with something like a bus that does pretty much nothing but stop and go in short cycles.

http://www.hydraulicspneumatics.com/200 ... 1985/Issue

Quote:

As for the regeneration charging the battery on a non-hybrid, I'd really like to see the math on *that* one. Don't normal alternators have significant drag even when the battery is basically fully charged? If so, I suppose you could add a clutch to disengage (like AC compressors have) and thus reduce the drag from the alternator when you didn't need it. *shrug* Still seems like yet another system that would take a LONG time to pay for the added complexity...

http://www.greencarcongress.com/2006/09 ... uces_.html

Richard Casto · **Posted:** Wed Mar 19, 2008 4:44 pm

So BMW (others as well?) are not running your alternator all of the time. While it might be "green" it also ups your HP numbers as well. Porsche has a variable speed oil pump in the Cayenne.

Traditionally you would spec out your oil system to generate enough pressure and flow at low RPM. Then at high RPM you generally have an excess of pressure as the speed of the oil pump is tied to the speed of the engine. That excess of pressure results in oil dumped back into the sump via a pressure relief value. This is wasted work that saps engine power at higher RPM. So Porsche now is doing a variable speed pump that keeps pressure up, but without generating high pump load at higher RPM.

Just another way to bump up your HP. Or appear to be "green".

Carl Fisher · **Posted:** Wed Mar 19, 2008 5:27 pm

The link I was looking for, but couldn't find, also talked about efforts along the lines that Richard mentioned: going to electric power steering pumps, water pumps, and even oil pumps to reduce high-rpm losses, and IIRC a small net decrease in total energy consumption (though how that could be, I don't understand)...

Michael Hankewycz · **Posted:** Wed Mar 19, 2008 7:55 pm

I wonder what the conclusion would have been had their experiment been done in 100% urban driving...

Regardless, it shows why the German manufacturers focus so much on diesel technology. I would love to see more diesel engine variants sold in the United States, but the appeal deminishes quite dramatically when you see what kind of premium one must pay for diesel here in the States (even when you consider the increased fuel economy).

How about a diesel hybrid? The Volkswagen Golf TDI Diesel

Donnie Barnes · **Posted:** Wed Mar 19, 2008 8:45 pm

Carl Fisher wrote:

http://www.hydraulicspneumatics.com/200 ... 1985/Issue

Ah, so I was right. The high pressure accumulator is really compressing nitrogen to "store" energy. Neat. It's annoying that the only way to figure that out from the article is the caption on the picture that's too small and fairly useless, though.

Quote:

http://www.greencarcongress.com/2006/09 ... uces_.html

Again, as I surmised.

As for the move to electric pumps on things, that's been going on in race cars, too.

--Donnie

Kevin Hoff · **Posted:** Wed Mar 19, 2008 9:47 pm

Electric pumps help for the same reason as the variable speed oil pump on the Cayenne. (How do they do that? Something like a CVT drive that adjusts from low to high reduction based on oil pressure?) All of the belt driven accessories have to work nicely at idle in a parking lot or customers complain so they get massively overdriven. Using electrical drive for accessories lets the system drive them the right speed at all times, saving power overall.

--Kevin H.

Pete Guthrie · **Posted:** Fri Mar 21, 2008 6:02 pm

Arthur McDonald · **Joined:** Fri Aug 29, 2003 1:54 am **Posts:** 2052

Chuck Branscomb wrote:

Quote:

To find out we set a challenge: to drive a Prius to Geneva using motorways and town driving. The direct route is 460 miles but we drove almost 100 miles further to give the Prius the advantage of running in urban conditions where its petrol-electric drivetrain comes into its own.

We took along a conventionally powered car – a diesel BMW executive saloon – for comparison and drove both cars an identical number of miles (545).

This is an inherently invalid comparison. Gas-electric hybrids are not designed to deliver top highway fuel economy - once the batteries discharge, they're just abnormally heavy cars with small displacement engines. (As a point of comparison, a Corolla has about the same passenger/cargo capacity - and better performance - than a Prius, but it weighs 400lbs less. Not to mention a sale price 25% less than the Prius.)

Not sure what to make of a BMW diesel in the U.S. From a TCO perspective, there are no BMW economy cars.

Kevin Butler · **Posted:** Mon Mar 24, 2008 10:21 am

I just returned from Ireland after a 2+ week stay. My associates and I all had diesel powered rental cars in which we were easily able to get 47 to 53 MPG.

The car I drove was one of the larger ones on the road, being a Ford Mondeo with a 6 speed manual transmission. My driving was about a 50-50 mix of in-town and highway driving.

Yeah, the numbers they post in the article sound real impressive to us here in the States, but in UK and Europe they're already able to do much better than the ranges addressed in the article.

The reason such diesel power isn't made available in the US is the particulate emissions in UK and Europe are not as stringent as what is allowable in the US.

Regardless of the larger particulate emission, Ireland was very green, even in February and is a very agricultural country. I'm wondering if it wouldn't serve the US to reduce it's emission requirements a bit, in order to make such fuel economies possible here.

Todd Breakey · **Posted:** Mon Mar 24, 2008 10:39 am

The problem with that is they *just* tightened the diesel emissions requirement last year so the liklyhood of them loosening the emissions are somewhere between slim and none. But all older cars are grandfathered in. So you don't have to put a urea scrubber on your 1980 VW Rabbit Diesel.

One of the things that has helped with diesel emmisions greatly was the new diesel requirement for Ultra Low Sulfur Diesel. This replaced the previous LSD last year. LSD was required to have less than 500 ppm sulfur, while ULSD is required to have less than 15 ppm. This was a huge step forward in bringing diesel emmisions down to an acceptable level.

If you really want to get real about why we don't have diesel cars here in america, look no further than our own General, General Motors. In the late 70's, GM introduced a diesel powered Cadillac Seville for, what was at the time, a staggering $20,000. It was bought readily by the public that was dealing with rising gas prices (sound familiar?) The problem was the engine that they used was nothing more than a gasoline engine converted to run on diesel. They were noisy, belched smoke like it was going out of style and were terribly unreliable. GM ended up by having to pay for multiple engine replacements on alot of cars. This put a sour taste in the mouths of the American consumer when it came to diesels. GM, FoMoCo and Chrysler never pursued the compact diesel for the US marked again.

When I started looking for a VW TDI last year, the first thing my parents said was "aren't they terribly unreliable?" This opinion was based on those GM diesels from 30 years ago. If the "Big 3" really want to get themselves turned around they would start bringing their small turbo diesels that they offer over in Europe. And then start touting the mechanical simplicity that is the modern turbo diesel. Hybrids are great but are filled with technology. What happens when the computer brain of a Prius pukes on itself while you are driving down the highway? I can guarentee that it will cost way more to replace that computer than it will cost to get a mechanic to work on your VW TDI.

Just my $.02.

CLICK HERE to read an article from 1983 about the GM diesel problem.

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Toyota Prius proves a gas guzzler in a race with the BMW 520

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