That's about what mine got on the highway. But city mileage could never get above 11-12. The truck could match that.
My speed vs mpg experiement
- Thread starter jayro88
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That's about what mine got on the highway. But city mileage could never get above 11-12. The truck could match that.
Which TDI do you have, and how are you getting 53 mpg? I generally keep to 65 to 70, and I generally do 45 to 47...
My brother used to be a engineer for Cummins. He said 1/4 more mpg was a big deal if you could get from somewhere.
david85
Full Access Member
Oh crap, you just let out my inner geak
It dates back to WW2 and even a little earlier. Engine designers realized they could cheat the requirement for larger displacement with a blower (super charger). That stuffed more air to burn more fuel, to make more power - simple. Then they realized efficiency dropped because they were taking power off the crank to drive the blower . Enter the turbocharger, which recovers small amounts of waste exhaust heat to do the same job. Now power and efficiency increased at the same time. The end goal of any engine is to convert heat into work, stuffing air into the engine is just something you have to do in order to get there, but what if you didn't have to stuff more air/fuel in?
Turbo Compound eliminates the compressor side of the turbocharger and uses a gear reduction to let the turbine directly drive the crankshaft. This reduces intake manifold pumping losses and avoids adding heat by compressing air inside the intake manifold. This in turn further increases power and efficiency by converting small amounts of waste exhaust heat directly into mechanical power at the crankshaft. Turbocharging can still be used in the same engine and most still go that route. It just depends on what the engine designer is after. The more power comes from the exhaust flow, the less forgiving the operating range can be, but the higher peak efficiency. Superchargers are less efficiency but generally offer unmatched engine response.
The allison V12 1710 engine went through all these stages during the course of WW2. Early versions were supercharged and put out less than 1000 hp. These were used in the P40, the anemic P39 and early versions of P51 (along with an ill fated batch of castrated P38s for export). The early turbocharged engines used in the Lockheed p38 lightning were limited at the factory to 1200hp due to fears intake manifold backfire. Later J and L model lightnings used larger chin scoop core type intercoolers that allowed the same allison V12s to reliably put out 1400 Hp at 100% and close to 1800 with emergency boost. They later found out earlier lightnings could put out more power too, but thats another story.
Toward the end of the war, Allison built one turbo compound engine that was bench tested to nearly 3000 hp. It proved the concept but by then it was the jet age and speed mattered more than range. Besides, turbo compound engines combine the complexity of piston and turbine engines into one power plant. This is the evolutionary missing link between piston and turbine engines but few have heard of it.
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The allison was a gasser, but the british napier nomad engine was diesel, and had a higher theoretical efficiency - better then turbofan engines of airliners today, but still much lower power to weight, so fuel savings would probably still favor the modern turbofan. Reliability of a modern turbofan is also hard to beat.
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Probably the best known application of a turbo compound engine, was the Lockheed super constellation airliner, which used radial piston turbo compound engines. In the Curtis Write R-3350, PRTs (power recovery turbines) took power from the exhaust, while a mechanical supercharger stuffed air in the front:
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http://www.superconstellation.org/TechnicalInformation/motor/motor-en.html
These days, the most common way they get power from the turbine into the engines is through the camshaft gear, but earlier engines had a separate gearing unit. A step down geartrain is still needed for diesels due to the RPM difference between ideal crankshaft and ideal turbine speed. Some guys have wondered if a Mazda rotary could be used to build a direct drive turbo comp, but I'm not yet aware of any one having pulled it off yet.
Manufacturers claim an improvement in efficiency of 3-5% across the operating range compared to conventionally turbocharged diesels. If reliability is the same, it should add up to some savings fairly quickly.
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FordGuy100
Registered User
Thats awesome!
People debate both sides, but my response would be "With a properly built one it can". I built one for my van, but have yet to be able to really test it out. In theory a properly built one doesn't (even though it looks like it does) increase the surface area of air that needs to move. It simply guides the air that normally would be dragging along the uneven surface under the vehicle and axles around the outside of the vehicle. Since it does it smoothly it does it with less drag created. Since I had scrap laying around and could build one for nothing I did. In my vans configuration at the time I did not see any noticeable gains or losses with it. Unfortunately, I never had the oppotunity to do any good tests on it. There were always too many other variables to get an accurate reading on mileage with it. I still have it sitting in my garage and may test it over the course of the next long road trip I take. Here are a couple of pics of it.
Before:
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