81 MPG golf TDI! ***Pre VW cheat scandal***

Yesterday I saw a Jetta TDI sportswagon and was wondering how it compared to the Golf:

	Metric	2014 Jetta SportWagen	2015 Golf SportWagen	2015 Prius v
1	Passenger	92 ft{3}	94 ft{3}	97 ft{3}
2	Luggage	33 ft{3}	30 ft{3}	34 ft{3}
3	City	29 MPG	31 MPG	44 MPG
4	Highway	39 MPG	42 MPG	40 MPG

Source: Fuel Economy

Impressive improvement in the Golf specs but I'd notice in the past VW had been under-reporting their TDI performance. Hopefully VW has figured out how to do the numbers 'less bad.'

Bob Wilson

 

The EPA test disfavors diesels in general. The Cruze diesel is rated 33mpg combined, but a quick Fuelly search has the user average at 41mpg. There is just a lot more diesel VWs to look at. The Cruze and BMWs are new, the MBs are expensive, and the heavy duty pick ups don't have an EPA figure to compare too.

The difference between the 2014 Jetta and 2015 Golf is likely emission controls. VW was using a NOx trap on the Jetta, which had it at MPG ratings close to the SCR equipped Passat. They've started using SCR on the smaller Golfs and Jetta.

 

Interesting, I had not really looked at the Cruze which I think is sized as Prius competition:

	metric	2.0 Diesel	1.4 turbo ECO	1.4 turbo	1.8 gas	Prius c	Prius
1	city MPG	27	26	26	22	53	51
2	hwy MPG	46	39	38	35	46	48

• City MPG - the 'turbo' really makes a big improvement in the Cruze
• Hwy MPG - the 'diesel' shines
• Prius City MPG - the Prius hybrid drive remains the champ . . . turning off the engine is a winner
• Prius Hwy MPG - the 'diesel' is competitive with the Prius
• Gasser Hwy MPG - the pump losses are real in the Cruze

These numbers look reasonable but what I'd really like to see is an Edmunds "Fuel Efficiency Smackdown." Take 3-4 cars on a three day road trip.

I like Fuelly as there are some embedded patterns in the data. But I suspect the diesel owners tend to have long, highway commutes which self-selects out of urban driving profiles.

Bob Wilson

 

The Cruze, Jetta, and Prius are all close in terms of cabin space.
The 2L diesel in the Cruze may have been selected as the engine to use because VW uses a 2L here.
With the current Cruze's weight, pushing the turbo into higher boost ans sucking more fuel is a little easy.

Diesel owners may self select, but isn't that also true of hybrid owners? Besides Cruze diesel listings at Fuelly are still above the city rating.

 

Agreed as the original mission profile in 1993-94 for the Prius was as an urban friendly vehicle: high efficiency and low emissions.

Bob Wilson

 

Indeed:

I took these photos today:








Bob Wilson

 

2015 Volkswagen Golf Sportwagen TDI Review – Video - HybridCars.com

 

This is as fine a place as any:

• variable intake valve delay vs open throttle plate
• expansion ratios
• internal friction
• cooled, recirculated exhaust
• topper cycles

Variable Intake Delay vs Open Throttle Plate

One of the diesel advantages is power throttle is handled by the amount of fuel injected versus the amount of fuel-air mixture in the cylinder. The delayed, Toyota intake close pushes part of the fuel-air mixture back into the intake manifold so the compression stroke is ~8:1 versus typical diesel of 15-20:1. Taken to its logical extreme, the variable inlet valve closing could eliminate the throttle plate. But unlike the diesel, this means the amount of energy needed to start the engine does not have to fight the high power needed for the diesel compression ignition to work.

Diesels take a significant amount of power to turn over because of the work needed to compress the first air charge enough to auto-ignite, even with a glow-plug. So one reason to keep a diesel running is to avoid the heavy starter draw on the battery. In contrast, the Prius engine spins up with a significantly lower energy requirement because the compression stroke is so low.

Expansion Ratios

Diesels typically have a higher expansion ratio than the Prius Atkinson ratio. The problem is this keeps the air-rich N{2} and O{2} at high temperatures where they can combine to make NOx compounds. It never really has a HCx rich exhaust that the Prius uses to reduce emissions with the catalytic converter. But there is no reason why the Prius could not have a higher expansion ratio. IMHO, this is the direction the next generation of Prius engines should go.

The problem is a higher expansion ratio also cools the exhaust temperature. The down-stream, catalytic converter requires heat to operate. It also requires a mix of lean and rich exhausts to chemically treat the NOx and HCx compounds. In effect, the catalytic converter needs heat to work. In contrast, diesels tend not to have similar catalytic converters. Their challenge is dealing with NOx and a different approach is needed.

Internal Friction

A primary friction source are the piston rings that have to seal the cylinders. They are typically pushed against the cylinder walls in part in proportion to the pressure inside the cylinder. The delayed intake valve closing and lower compression stroke ratio reduces this friction. In contrast, the diesels have to seal with a larger compression ratio and higher ring friction.

Cooled Recirculated Exhaust

This diesel technique was borrowed for the Prius engine at high power settings and it works great. For the Prius, the problem is the down-stream, catalytic converter that can be damaged with higher temperatures. So using exhaust gas mixed with the fuel-air mixture reduces the combustion temperature saving both NOx formation. This is a technique for high-power settings when gas flow is needed but not the heat.

As for diesels, the cooled combustion helps mitigate the NOx formation. It doesn't eliminate it but reduces the amount needed for down-stream processing.

Topper Cycles

Not seen in cars, one of tricks used with high efficiency, diesels are topper cycles that extract energy from the exhaust stream. There have been attempts to do this with cars but nothing I've seen in production. However, there is the exhaust heat recovery system in the Prius used to supply heated coolant for rapid engine warm-up. In theory, this can also provide cabin heat but I've not seen this in North Alabama with our 2010 Prius.

Today, our Prius consumes about 350-450 W in normal electrical overhead. This overhead is within the range that should be recoverable from exhaust heat with a topper cycle. But living south of the Mason Dixon line, I would prefer to see an absorption cooling system tapping the exhaust heat to cool the cabin.

Conclusion

Higher expansion ratio along with a broader range of intake valve closing delays is the path to higher Prius engine efficiencies. Using cooled exhaust saves the catalytic converter so the multi-way chemical processes can reduce NOx and HCx by an order of magnitude. The last element missing is exhaust heat recovery beyond just warming the engine.

Bob Wilson

 

There has been some work on compression-ignition (diesel) engines using gasoline as the fuel. Preliminary tests indicate fuel consumption of 175 g/kWh (somewhere around 48% thermal efficiency?).

Green Car Congress: Delphi advancing Gasoline Direct-Injection Compression-Ignition engine concept; new two-stage supercharger/turbocharger boost system

See also Hyundai twin-charged 1.8L GDCI gas engine expected to be more efficient than 2.0L diesel [UPDATE] and Hyundai Developing Gasoline-Burning Compression-Ignition Engine – News – Car and Driver | Car and Driver Blog.

This concept is apparently different than the HCCI concept in that the gasoline is injected near TDC and auto-ignited similar to a diesel engine.

 

... two things in life everybody lies, straight through their teeth:

1). pissing in the community swimming pool --
2). fuel consumption --

I must be the only one who doesn't.

Having exclaimed soon after buying my Prius, I get 34 mpg driving it like a Corvette, my wonderful next door neighbors, God bless their hearts, never to be outdone, lying straight through their teeth in the great American game of one-upsmanship, retorted in a huff something's definitely wrong with me because, they're averaging well over 40, in town, in their 6 cylinder Avalon. And, several occasions, they've had it over 180 mph.

"Oh, absolutely! No two ways about it, the Toyota Avalon, fastest car in the world; best fuel mileage! How could my Prius ever hold a candle to your remarkable Avalon?" Be agreeable; humor the silly old folks; no sense being argumentative. Right?

I drive my Prius, sometimes my Subaru -- or, when I have certain business behind the wheel, my heavily modified MR2 Supercharged -- across a 40 mile stretch of canyon road rising twice beyond 4 thousand feet elevation, on a weekly basis for several years, where I never see more than 34 MPG in the Prius; 18 in the Outback; down to single digits in the two-seater.

Yet, lying straight through their teeth like everybody else does, the good folks of that hard hitting publication, Sport Compact Car Magazine, across that very stretch of asphalt I drive, boasted 30.4 MPG in a 340 horsepower 3.6 liter mid-engined Ferrari? A stunning 32.2 mpg, in an over-modified 500 hp twin turbocharged Supra? And, an astonishing 36.1 mpg, in an even more heavily modified HKS twin-compressor (e.g., turbocharged AND supercharged) MR2?

Behold, the depth, breadth and extent even reputable automotive journalists lie, about fuel consumption:

2002 Ultimate Street Car Challenge results - Wikipedia, the free encyclopedia

In actuality, downhill with a tailwind, no Ferrari in history ever got 20 mpg, much less 30. No Supra, no supercharged MR2 in history, ever got close to 30. To further underline the extent to which automotive journalists lie, toggle back to the link above, scroll across to the Ferrari F360's recorded lap at Willow Springs Raceway of 61.8 seconds, a full 4 seconds per lap faster than Michael Andretti's track record, in an Indycar? See for yourself:

Track Records | Willow Springs International Raceway | Since 1953

"Oh, so cheating its emissions asunder, the Golf TDI gets 81 mpg? I want to thank you for setting me straight." Things are so bad in contemporary car culture, even the Volkswagen people are strangers to the truth.

Fortune cookie logic for your message board amusement and merriment: "... never swim in community pool with Volkswagen people."


- Samuel, '04 Ruthiemobile

 

Calm my friend. There are so many variables and some are completely unknown to the owner. I'm thinking of the non-technical folks who complain about the Prius mileage yet won't life a finger to investigate the symptoms. Intellectually lazy, loud-mouths, I get tired of them.

Bob Wilson

 

poor wayne, duped by the dub.

 

About 480,000 'waynes' based upon the VW press release.

Bob Wilson

 

Bob,

This is a great explanation. I wish I fully could grasp it. I get some, but not all of it. I do better with animation showing the process.

When I was a teenager turning wrenches on my first car (1966 Chevy Impala, 2 speed auto) the cars were so simple. I swapped transmissions, replaced cam shafts, etc.

The high tech of modern cars is really cool, but I'm just a parts guy. I can remove a part and replace it. I thought the first engine I totally rebuild was easy. Just like baking a cake, just follow the instructions.

I'm right on the edge of fully understanding the engineering you describe here.

Thanks

 

Just highlight the text in the quoted parts that I didn't explain very well and I would be happy to translate 'the Bob'. Others have pointed out my Engineerlish is not always well understandable. <grins>

Bob Wilson

 

I understand how a heat pump can cool or warm a house, but I don't understand how well exhaust could be used to cool the cabin. It sounds like a Rube Goldberg machine. It could be done, but at what complexity?

In the aircraft I fly we use an air cycle unit (pack) to condition the air. It's clean, hot, compressed air that turns a turbine that rapidly expands to near freezing. The near freezing air mixes with hot air through a mixing valve that outputs nice, conditioned air for the cabin and pressurization.

I suppose an exhaust turbocharger could turn a compressor to create clean, hot compressed air to a "pack" for the Prius, that's makes cold air.

It just sounds expensive and complicated. How do you think that would work? Are you thinking something totally different than an air cycle unit?

Cheers

 

Yeah 81 mpg is possible! But not under normal driving conditions for a every day driver! So what! Doubling your travel time for a few bucks! Hardly worth it!

 

They are called "aborption refrigerators" and propane refrigerators use this cycle. There is a basic chemistry test I remember vividly:

A closed flask filled with ammonia is inverted so the mouth is in a beaker of water. "FaWomp" the water is sucked into the flask and its temperature drops. Dissolving ammonia in water chills the system.

Now if you heat the ammonia-water solution, it will drive the ammonia out. Cool the ammonia and water in separate loops, a pair of radiators. They also dump the heat that warmed the solution.

Once cooled, the water and ammonia can be combined again to make cold, ammonia-water solution that can be used to cool the cabin.

There are complications:

1. Ammonia eats many metals - very reactive.
2. Water freezes - do not let the water tank crack (there are flexible tanks!)
3. Everything has weight and the thermodynamic efficiency means you need a lot of free, low-grade heat . . . like an engine exhaust

A similar reaction can be done with ammonium chloride:

BTW, aluminum tube is ammonia safe.

Bob Wilson

ps. Don't cha' love thread drift that ends well?

 

FWIW, absorption cycle chillers are also used in some buildings. Applications include:

• Using "free" waste heat from an adjacent industrial process or on site electricity generation
• Meeting critical cooling loads when mains power fails (eg Operating Room cooling in a hospital)
• Handling short term peaks in the cooling load where electrical demand charges are high