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Direct Shift Gearbox (DSG) Transmission by phil

Posted in New Car, science on June 11, 2011| 1 Comment »

This is the first of many posts outlining specific reasons why our new Jetta TDI is awesome.  One of the main factors in our new car search was gas mileage.  The EPA estimate for the Jetta TDI is 43mpg and on a recent trip from New Jersey to Maryland I averaged 44.43mpg. In this post I am going to explain one of several pieces of technology which contribute to this type of fuel efficiency.

The Direct Shift Gearbox (DSG) transmission is distinct from the transmission you have in your car unless you drive a VW or Audi (or Porsche, except they call it a PDK but it is essentially the same design). The DSG is essentially a manual transmission which is shifted automatically. This may seem trivial but there are significant differences between a standard manual and a standard automatic transmission.

I could dedicate pages to explaining, comparing, and contrasting in detail these two general types of transmissions but people MUCH smarter than I have already done so.  I am, however,  going to try to give a quick overview of each in order to highlight how cool our transmission is.

Standard Manual Transmission

For any transmission the goal is to apply the optimal amount of horsepower and torque at a given rpm. In a standard 5 speed manual transmission there are 5 different horsepower/torque combinations which are manually selected by the driver. The picture below illustrates a very simple two-speed transmission.

Simple 2 speed manual transmission

In the above 2 speed manual transmission, the purple gear selector fork can be moved forward or backward to engage the small or the large blue gear. The different diameters of these blue gears (and associated red gears) are what applies the different amounts of torque to the gear shaft. This is easy to visualize for a two-speed manual transmission but slightly harder to visualize for a standard 5 speed manual transmission, see below.

Slightly more complicated 5 speed transmission

As you can tell its more complicated but all the parts are the same. If you look at 1st gear you can see how that would provide significantly more low end torque than 5th gear.

Automatic Transmission

A standard automatic transmission still applies torque but goes about it in a mechanically different way. A typical standard transmission uses whats called a ‘planetary gearset’ to apply different amounts of torque. A simple planetary gearset consists of a sun gear, planet gear, and the ring gear. The following illustration makes the naming convention slightly more obvious. Note, this not exactly how the gears look in an automatic transmission, it is more to get a visual of how this gearing structure works.

Simple Planetary Gearset

Appropriately the  yellow gear is the sun gear, the red gears are the planet gears, and the blue is the ring gear. This is a little harder to visualize but the different amounts of torque come from applying input power on one gear shaft (yes Jared/Akers I said shaft) and taking the output power off another shaft. Another wrinkle is that planetary gearsets have a mechanism to hold one gear stationary while the others continue to rotate.

To make this a little more obvious consider the following two scenarios.

First, consider the input applied to the sun gear and the output taken off the ring gear with the planet gears held stationary. In this scenario the output ring gear would be rotating providing a specific torque.

Next, consider the input attached to the ring gear, sun gear held stationary, and the output taken from the planet gears. This scenario would output a very different torque as compared to the first scenario.

Below is another drawing of this idea. The 3D representation helped me understand a little better. The bright red marks can be used to calculate the gear ratios.

So, if you followed what I was saying above, you understand how a simple planetary gearset works. Unfortunately, in real life the gearset looks like the two figures below and instead of one set of sun, planet, and ring gears, there are two. This is necessary to achieve all the required torque settings. It’s complicated and I’m not sure I understand all of it but the mechanical engineers that came up with these designs are god damn geniuses so I am sure it works.

Planetary Gear Set Transmission Pulled Apart

Planetary Gear Set Internals

Direct Shift Gearbox (DSG) Transmission

This brings me to the most interesting thing I have written about to date on this blog (aside from the chicken blood of course). The gear box in our brand new 2011 Jetta TDI is an automatically shifted manual 6 speed dual clutch. So, compared to the first part of this post the physical mechanism which applies different amounts of torque is most like a manual transmission but there is essentially a computer which ‘manually’ shifts the gears based on the engine RPM and gas pedal input. This combination basically gives us the manual gearbox advantages while minimizing the automatic gearbox negatives. Another way in which the DSG transmission behaves like a manual transmission is when you are not engaging the gas or the break. At rest if you take your foot off th break the car will not inch forward like an automatic transmission would. While moving if you take your foot off the gas to coast, you will experience engine breaking, just like a standard manual transmission.

Manual 6 Speed Transmission VS. DSG Transmission: The manual transmission boasts a lighter weight, less parasitic power loss, and slightly better gas mileage (mpg) if used efficiently.  It should be obvious how these first two advantages lead directly to better gas mileage. However, the difference in fuel efficiency is typically on the order of 2-4mpg, not a huge jump but it equates to legitimate fuel savings.  The main advantage of the DSG transmission is ease of use. All shifts are made automatically, just like any standard automatic transmission. One thing that helps the DSG transmission in terms of fuel efficiency is the precision in how it shifts in turbo diesel. Since everything is done by computer power is delivered to the wheels in a much more consistent manner than with a manual transmission.  This is because the computer keeps interruptions in power delivery between shifts to an absolute minimum, thereby keeping the turbo spooled and maximizing fuel efficiency.

My reasons for writing this post were three-fold. I will now explain them in order of importance.

1) I understood the transmission in our car was basically an automatically shifted manual transmission but that is all I knew. Holy crap, I learned a lot about this transmission. Coincidentally, holy crap did we make a good choice on transmission.

2) Initially, I had a surface understanding how manual/automatic transmissions worked. I really wanted to understand at a fundamental level how any engines translate usable power so well and at such a wide variety of speeds/weights/rpms/torques/etc. It honestly still amazes me. Now, when I accelerate from every stop I find myself paying more attention to the transmission progressing through the gears and turbo spooling when I should really be paying attention to the road or other drivers around me.

3) I really wanted to find a way I could legitimately use the word ‘shaft’ over and over.

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Virgin New Car Buying by phil

Posted in New Car on February 2, 2011| 4 Comments »

After months of research Amy and I finally pulled the trigger on a new car. We decided to go with a brand spanking new VW Jetta TDI. The TDI specification indicates that this particular model has a turbo charged diesel engine. In the past, diesel engines typically conjour up visions of big, loud, and dirty. I can assure you this is not the case with the newest class of diesel engine from VW.  In future posts I will outline the specific techniques VW employes to make this such an efficient and clean engine. Right now I just want to concentrate on the pure ecstasy that is driving a new car.

As you can see we went with ‘Platinum Gray Metallic’ (or just gray as I call it) and ‘Titan Black’ leatherette interior. The 2011 model has a slightly more mature look than the 2010 model. To steal a line from Jimi “It looks less like a toy and more like a car”. Overall I like the look a lot, I didnt really care much about the colors but I think Amy and I jointly picked a good combination. I took a panoramic video of the car embedded at the bottom of this post.

As I said, we chose a brand new 2011 which was delivered directly to the dealership. Because of this it only had 11miles when we drove it off the lot. I’m not sure the miles really matter but it was kind of cool driving away with something that had not even been driven across town. If you were wondering, yes, it was 15 degrees out when we picked up the car.

As we were researching different models the main thing we were looking for was something with good gas mileage. We went into the car buying process expecting we would buy a (possibly used) hybrid. Gas mileage is important to us because this is the car which I will be driving 30 highway miles between Baltimore and the Aberdeen Proving Grounds when we move there late this summer. After test driving quite a few hybrids we came away with the impression that most had relatively small interiors (which we are expecting to fill the back seat in the near future) and all had very small trunks. The small trunk is on account of the massive batteries which must be concealed somewhere.

Upon doing more research we discovered that the fuel efficiency of these diesel engines rivaled hybrids especially on the highway. Since my commute would primarily be on the highway this seemed like a good match. Additionally these engines employ some very sophisticated technologies to keep the fuel consumption to a minimum. Our Jetta TDI gets an EPA estimated 43mpg on the highway but I have found people on the internet reporting closer to 45-46mpg  in real world driving. As a comparison, a 2011 Prius gets an EPA estimated 48mpg on the highway. Additonally, as the Prius 4 cylinder engine is pushed faster than 55-60mph the gas mileage decreases. Needless to say the turbo diesel does not suffer from this problem at high speeds. Looking at the previous and the following pictures you can calculate the fuel consumption if you know our gas tank holds 14.5 gallons. Based on those pictures the calculation comes out to 50mpg, this is probably not a good estimate because it is only the first 1/8 of a tank and I have found a pretty heavy foot driving to work lately.

There is a manual transmission available which would provide even better fuel consumption but Amy and I opted for the automatic. Our transmission is not a standard planetary gearset automatic transmission, I will have a post dedicated to how cool the transmission is in the future. For now just know it’s an automatic and its awesome.

The worst part of buying the car was probably dealing with all the asshole sales people. What pisses me off is that they would always give us their ‘absolute lowest price’ where I am ‘robbing them blind’, then when I found a cheaper price somewhere else they would have no problem matching it. The first price was obviously not their lowest, why lie to me? Either way, let me tell you how Amy’s dad and consumer reports saved us about $2000.

Tom Watkins has a subscription to the consumer reports magazine and website. This is what we used to do the majority of our research. After a few days of haggling with sales people we found a feature of the consumer reports website where we could get quotes from dealerships in our area. Once we requested quotes through consumer reports we had 4 salespeople call us within 30mins. Now that they knew we were serious and that they were competing with other dealerships they actually tried to win our business by significantly lowering the price and accommodating any other needs we had with the car buying experience. As an example, we were initially told they could only hold a car for us for 2-3 days. Once we went through consumer reports they had no problem holding it for the rest of the month. That is the kind of stuff I hate, tell me the truth from beginning and we will have no problem. Overall, we saved $2250 and got much better customer service by going though consumer reports. Moral of the story, if you are in the market for a new car spring for a consumer reports membership, it could save you thousands of dollars.

I have a whole lot more to say about this new car but for now I will leave you with my current fuel efficiency and a reverse panoramic video of the car taken with my droid X  which Akers personally programmed for me. Notice my gas mileage has decreased from approximately 50mpg over the first 90 miles to about 35mpg over the first 250 miles. The reason for this is that I got much much more comfortable with the available torque off the line and using the torque to make scary dangerous necessary passes on the highway on the way to work.

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