Propane Fueled School Bus

[akirby]

 

And of course, if it's just a wash, there's no point in adding the cost/complexity

 

Well it would give you the ability to vary the valve timing independently or shut off the valves completely (cylinder de-activation)? Not sure how much benefit that provides or whether it's worth the complexity trade-off.

 

It also means a software glitch or pcm failure could destroy an interference engine in a few milliseconds.

[lfeg]

Re the electric hybrid/turbocharger, That is on the new Formula 1 engines. The turbocharger incorporates a motor/generator so the turbo can generate electricity when it is recovering more energy than needed to compress charge air. Conversely, when exhaust flow is to low to provide enough energy to properly compress the charge air demanded, the M/G can power the turbo. In practice electricity produced by the turbo and be used to power the main M/G in a hybrid drivetrain.

 

This gets me thinking back to (I think it was Napier) when a turbocompounded diesel aero engine was produced. It ended up that so much power was being taken off from the turbos that the basic diesel engine itself acted more as a fairly inefficient combustor for what was essentially a gas turbine. I will have to find my references on that.

[lfeg]

And as to valves, with the materials we have today, along with our better understanding of lubrication and surface interactions, how about just eliminating the poppet valves? Rotary valves such as the Knight and Cross designs (in the 20s and 30s) and the Aspin designs (50s I think) did work, but were limited by the materials and processes of the times. Such designs may be electrically operated easier than poppet valves.

[Edstock]

This gets me thinking back to (I think it was Napier) when a turbocompounded diesel aero engine was produced. It ended up that so much power was being taken off from the turbos that the basic diesel engine itself acted more as a fairly inefficient combustor for what was essentially a gas turbine. I will have to find my references on that.

 

IIRC, Curtis-Wright made turbo-compound engines from some of their radials for use in post-WW2 airliners, like the DC-7 and the Super Constellation. Unlike standard turbo engines, the turbocharger had a mechanical connection to the engine output shaft.

 

Amazing that they were able to make it work and keep flight schedules.

[lfeg]

Yes, the Curtis-Wright turbocompound engines did work well. They used a differential gearbox for the mechanical connection if I remember right. It was a rather robust design. I am going to have to dig out the book I have that discusses turbocompounding, as one engine builder (and I think it was Napier) went so far with turbocompounding with multiple stages that they injected fuel into the exhaust stream before the last stage to use up what little oxygen was left to generate more heat and exhaust volume. The number that I remember is that over 70% of the shaft horsepower came from the turbocompounding! (and that it was a diesel aero engine made it even more interesting.)

 

So, to get back to topic, should we be discussing an Ecoboost turbocompounded engine for school busses?

[old_fairmont_wagon]

My father had the pleasure (displeasure, depending on which day you talked to him about it) of turning a wrench on those 3350-TurboCompound engines when they were in use by some of Delta Airlines' fleet. The competing engine was the 4 row P&W 4360 of more conventional turbo-supercharger design. They each had their own headaches and problems. The 4360 was an extremely powerful engine, and the culmination of P&Ws piston engine technology (with even one version using variable discharge turbos to provide additional thrust from the exhaust). The main drawback of the 4360, and something for which no fully complete solution was ever found, was that the 4360, as designed, was air cooled. The issue was that, being a 4 row engine, the air was already superheated by the time it got back to the rear row of cylinders (depending on which way the engine was mounted (the B-36 had them mounted backwards in a pusher configuration) and, as a result, the last row of cylinders suffered from thermally caused fatigue and degradation.

The 3350 turbo-compound with the PRT as it was used in commercial aviation was its own maintenance nightmare. Besides being more complex than the more traditional design of the P&W 4360, there were issues with vibration in the engine introduced by the PRT as well as the problems caused by the elevated EGTs that the back pressure generated by the PRT caused. As in the 4360, this resulted in cooling problems that shortened engine life. In the last few generations, some materials were improved to more exotic (for the era) types, which helped to some degree, but, in the end, both engine designs were pushing the limits of piston engine technology with respect to power/weight and to absolute output power on a common shaft. The 3350 was definitely the more efficient engine of the two, but it was more expensive to operate in every other phase of its life, including procurement, maintenance, and overhauls.

To give a comparison, the Boeing 377 would commonly come in with all four engine still working, but the flight engineer would produce a report of instrument readings and vibration on each of the engines that would indicate that It had a handful of cylinders that weren't operating at any given time across all 4 engines. The Connie's would come in often with a whole engine dead and a feathered prop (assuming the engine hadn't just seized and wring the prop off in some random direction in the process). You could get that 377 back in the air within an hour with almost all of the cylinders working again with 4 mechanics (you may have fixed all the dead ones, but, with 128 cylinders on the plane, you'd likely loose a couple in startup). You MIGHT get the connie back in the air the next day with 4 mechanics and a spare engine to swap in.

Now, getting back on topic, turbo-compounding is a technique that lends itself much better to engines that run the same RPM for VERY long periods of time. I don't think that you'd want to run a 3.7L Turbo-Compound V-6 in place of a proper 7.4L V-10. Though, a twin turbo 6.8L V-10 would be a very interesting animal indeed.

[7Mary3]

I think the turbo-compound R-3350's had some sort of a fluid coupling between the PRT shafts and the crank. Impressive engine, but notice the Connies and DC-7's so equipped didn't stay in service as long as the DC-6 did with it's conventional P&W R-2800's. As for as automotive applications, I believe Detroit Diesel has turbo-compounding on one of it's larger highway diesels.

 

EMD uses an interesting turbocharger on it's 2 stroke railway diesels. The turbo is driven by a shaft from the timing gears at low r.p.m.'s to proved the positive pressure required for the engine to run, but as r.p.m.'s increase the exhaust spins the turbo up and an overrunning clutch disconnects the impeller from the timing gears.

 

Not seeing an application for Ecoboost in a commercial truck, I think the duty cycle is such that there would be no advantage to it.

Edited September 9, 2014 by 7Mary3

[old_fairmont_wagon]

The Connies and DC-7s (and even the Boeing 377) were retired quickly because they were rapidly eclipsed by the first generation of jet airliners. The first Douglas DC-8 was delivered about 5 years after the first DC-7 took to the skies. The Boeing 707 (and its variants) were being delivered within the same timeframe. Convair had the 880 in the air also at the same time. The turbo-compound 3350s (both with and without PRTs) served long lives in the freight business. First generation jets were gas hogs and since, at the time, freight was rarely that time sensitive that a few additional hours in transit would really matter, the MUCH lower fuel consumption of the 3350s made them very economical cargo aircraft and many remained in use through the 1980s and even a few into the 90s. Over the long haul, the 3350 once improved with parts made from better materials and with improved cooling designs, proved to be more economical to operate than the Wasp Major 4360 in the cargo world. The Wasp Major was pushing the size of radial piston engines too far. Too many cylinders, too many plugs, too much heat to dissipate in an air cooled engine. One only has to look at the usage of the engines over time. The biggest single factor was that almost every 4360 that wasn't in specialized use was parked soon after the spike in fuel prices in the mid 70s. The 3350's were more efficient in fuel usage.