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to replace a diesel

Posted by ReubenT 
to replace a diesel
February 27, 2016 09:55PM
Lets design a steam engine and boiler setup that might replace a diesel in heavy equipment. Track loader or dozer has limited engine compartment, it would be a challenge to make a boiler small enough to fit along with the engine, and big enough to do the job with reasonable speed, a track hoe would have more room. dozer and loaders need shaft drive as well as pump drive, the hoe only needs pump drive. They generally run at around 2000 RPM. The first candidate is a case 450 loader that kicked a rod through the side of it's engine. I put a V6 gas engine in it but it's not the best. I could potentially patch up the diesel engine, but not sure I want to. Engine is a 188D 2100 top RPM, 150 ft lb torque.
My brother and I now own half interest in a komatsu track hoe, but it's not a candidate since it's in good shape. with some extra cash to play with I could buy an old one to work on. We almost had one already, early 70's international that needed lots of work, engine rebuild, hydraulic cyls rebuilt etc. But it got away. Once a basic design is worked out it could be scaled or modified a bit to fit many applications. I could see it being quite practical.

Boiler would be dual fired for practicality, either waste oil or solid fuel.

Steam is notoriously slow moving. Even modern single acting V engines are rated at 1000 RPM. Can a steam engine be designed to go up to 2000? or would it be better to run a higher torque engine slower and gear it up. That presents issue with gearbox or chain drive, extra bulk, expense, or limited wear time. I'm thinking a 2 cy, double acting design, both in one casting perhaps, variable cutoff inlet valves with shaft governor controlling them. Hand lever to set the governed speed. Just checked again and found Peter has a V4 single acting compound that will go to 1500 RPM and 50-125 hp. Perhaps that would be better, it's getting close to what's needed. Since the double acting needs extra link arm and crosshead which weight would slow it down. Any ideas?
Re: to replace a diesel
February 28, 2016 09:17AM
There is nothing about steam that makes an engine inherently slow, else steam turbines wouldn't have such high tip velocities. The PSL engine Chuk used in the streamliner was rated somewhere around 2,000 revs by Art Gardiner and Chuk often exceeded that; personally, I think Art was a bit conservative in his calculations due to his long background with commercial Diesels although that may be more on point here.

The two big problems I see are due to basic mechanical engineering choices found in the majority of steam engines:
  • Reciprocating Mass
  • Valve Arrangements

Reciprocating mass is an obvious issue; the heavier the part, the more force it will generate on the engine structure. These forces go up as the square of RPM, making increased mass AND speed a one-two punch to the jaw.

Valve arrangements can have great impact on rated speed and are the reason Art specified a relatively low RPM limit on Chuk. It's acceleration that really limits a valve mechanisms performance and, depending on the type of valve, the desired cutoff. The problems are minimal until you start trying to build a relatively efficient, powerful and compact engine. Thermodynamically, higher pressure and temperature will theoretically yield better efficiency; this can only be achieved by cutting off steam admission quickly and allowing the steam in the cylinder to fully expand. There's where the problems lie. As the cutoff is shortened, the time the valve has to open and close are reduced. If you increase the RPM, the operating period becomes even shorter. Theoretically, a piston valve has some advantages, since it is in constant travel throughout the stroke it also has manageable levels of acceleration. On the other hand, such harmonic valves have strong weakness in the areas of lubrication and sealing. By contrast, poppet valves need little to no lubrication and seal exceedingly well. The problem is that they cannot be moved throughout the stroke but only during the operating period. This is no hardship on the exhaust valve, which must be open for a full stroke, but can be challenging on the admission valve---it must be open only a brief period as contrasted to an ICE admission valve which has an open duration similar to the exhaust valve.

The above is an Achille's heel of many "advanced" steam engines, the designers assume that the valve can adequately respond to whatever cam profile they create. Last time I checked, the auto industry limits for valve acceleration (measured in "gravities" or "gees"winking smiley was about 450 for pushrod systems and about 750 for overhead cams. Overhead cams would appear to have an advantage any time valve operating duration is to be minimized. It should be noted that simply "carving a cam" is almost impossible; a cam that looks good to the eye will probably have rapid variations in acceleration and undoubtedly contain transients of excessive acceleration. Art used a "polydyne" profile on his cam, which is an extremely good concept which has been replaced by the very similar "polynomial" profile, in particular the "4-5-6-7 polynomial" design. This design not only holds acceleration to the design limits but also creates smooth changes in acceleration so as to minimize shock to the valve train.

One possibility of reducing acceleration is to admit steam to the cylinder longer, this being a poor solution if high efficiency is desired. Needless to say, by compounding the cylinders, one can sneak by with longer cutoff but still achieve decent expansion ratios. Mechanically speaking, however, compounding by adding piston stages has some serious liabilities. Personally, I think it wise to consider a radial inflow turbine for a second stage. That still brings a considerable problem meshing two expanders running at such different speeds, especially given the forces that would be applied to the turbine by sudden rapid changes of speed in the piston expander. The simplest solution is to employ the turbine as the motive power for an auxiliary package containing feed pumps, blowers, condensate pumps and so on. Removing these loads from the piston engine frees up its horsepower for primary mission requirements yet still keeps efficiency high by utilizing a greater portion of the steam's energy.

The double-acting engine seems to possess too much reciprocating mass to really favor high RPMs. On the other hand, the piston rod seal and crosshead can serve to isolate the steam cylinder from the crankcase --- preventing piston ring blowby from reaching the lubricating oil. Various coping mechanisms for removing the blowby from the oil, such as centrifuges, have been tried and found successful. Another option is given us by Allen C. Staley. He isn't that well known but was a respected engineer at one time or another associated with the Doble-Detroit, Stanley, Scott-Newcomb, Coats and Endurance steamers --- in addition to the steamer he built as a Mechanical Engineering professor at Purdue. He's mostly known for his later work in air conditioning, supercharging and gas turbines at the Chrysler Corporation. Basically he designed a piston with an upper sealing surface to hold steam in the cylinder, a lower to keep blowby out of the crankcase and uniflow exhaust in between to vent the exhaust steam and blowby out of the cylinder so as to prevent any pressure buildup on the lower seal. This was used in his Purdue car and also copied by Henschel after WW2. See attachments.

I foresee some serious difficulties placing the engine, auxiliaries AND boiler into the same space previously occupied by a Diesel of similar horsepower. These go up further as you try to incorporate solid fuel firing, simply installing a fuel hopper and stoker is going to add a great deal of bulk.

Regards,

Ken



Edited 1 time(s). Last edit at 02/28/2016 09:14PM by frustrated.


Re: to replace a diesel
February 28, 2016 07:56PM
Definitely some challenges. Food for thought. I will be thinking, there just might be a way. Will need to do experimental engines. To use one commercially oil fired would be almost necessary, takes too much time handling firewood. But home use is far less demanding timewise, and refueling frequently with firewood could be managed. (since we'd be dealing with trees constantly anyway) The case 450 might have room for a yarrow boiler and compact 2 cy vertical engine behind it, or a 4 cy V, feed door where the grill is. I think I'd build one to fit and test it outside the machine, and if it looks like it'll do it, install it, if not, use it on something else.
Re: to replace a diesel
February 29, 2016 02:11PM
frustrated Wrote:
-------------------------------------------------------
> Mechanically speaking,
> however, compounding by adding piston stages has
> some serious liabilities.

A couple notes on this, one main upside of compounding is that for the same work/expansion ratio, the piston's supporting components need to handle 3 times the load uncompounded. The counter to this is more seal losses in the compound. In general they appear similar in efficiency.

What liabilities do you foresee Ken?

I like the finishing turbine also, downside is you need several cylinders to provide even a remotely uniform input pressure.

Cheers,
Keith
Re: to replace a diesel
February 29, 2016 05:26PM
Hi Keith,

Perhaps the most serious mechanical liability found in compounding is that the long cutoffs make for high Mean Effective Pressure in the HP cylinder. Blowby is a function of not only the pressure but the duration of the pressure application. If the high pressure duration is very brief, the flow stops before there has been significant leakage past the rings. The cutoff is going to vary inversely with the number of stages, a simple expansion having the shortest cutoff and the lowest MEP ... and therefore the least leakage. I originally theorized this and was happy to find Jay Carter telling me the same thing. Then Tom Kimmel loaned me the Dutcher Industries report and they described the phenomenon in pretty great detail. So, right off the bat, compounding is already suffering a real loss compared to the theoretical outcome.

The next problem is that the volume of each succeeding stage has to be greater, which almost invariably involves increasing the cylinder diameter. If the cylinder is twice as large, we can take it for granted that the friction will also be about double. On the other hand, we'd like to arrange for each stage to deliver about the same power. If the power is about equal and the friction goes up in a linear fashion, we are going to find ourselves attaining lower efficiency for each stage. Any way we look at it, a 2 or 3 cylinder compound should have more friction than a 2 or 3 cylinder simple expansion engine. The same applies to the cylinder surface areas, the compound has proportionately greater area and therefore more surface from which heat can be radiated and lost to the engine.

It's a given at this point that between the progressively larger cylinder bores and increasing reciprocating mass, we are going to have a significantly larger engine. Fitting a steam plant into machinery designed for much more compact IC engines is already going to be a challenge, this is going to add to it.

Steam somehow has to get from cylinder to cylinder; if you look at the steam mass and required acceleration it appears that a fair amount of energy is required to move the steam from one cylinder to the next. This energy comes from the interstage pressure drop; if the pressure has to drop to transfer the steam it certainly isn't going to be available for work in the next stage. That last statement does NOT hold true if the next stage is a turbine. Turbines function by converting steam pressure into velocity and then extracting work from the kinetic energy of the steam. Pressure lost between the cylinder and turbine is an integral part of the turbine operation and not a loss.

Another problem encountered with moving this steam around is the difficulties in obtaining smooth flow, turbulence will cost. And, of course, the transfer piping is subject to losing heat to the environment.

Then there is the whole problem of reciprocating mass. We want to eliminate double acting pistons to get the weight down but compound pistons are simply a lot heavier. This is going to have a negative effect on stresses and will require decreasing engine speed; this reduction in speed demands a bit bigger engine which in turn adds some more stresses requiring a bit more speed reduction.... Another issue that is going to rear its ugly head is dynamic balance. In a multicylinder, simple expansion engine, we can minimize shaking forces with careful cylinder and crankshaft layout; the compound is not going to balance anywhere near as readily for two reasons. 1. The most obvious to the casual observer is that the lower pressure pistons are heavier. Number 2 is a bit harder to recognize immediately, the larger cylinder diameter causes uneven crankpin spacing, in turn this creates differential leverage of the different pistons along the length of the crank --- exacerbating balance issues further.

Earlier mentioned was that we wanted equal power per cylinder; this results in a smoother turning moment and fewer stresses and strains on the machinery along with steadier output speeds. In a multicylinder simple expansion engine, this isn't even a concern, identical cylinder design gives you just that result. In a compound, however, both admission pressure AND cutoff affect power output per cylinder. Suppose the engine is running at a given pressure and the cutoff in each stage is set so that the power output is even distributed, that's great, right? Now suppose we throttle the pressure down. This means the steam cannot expand as much before expending its useful energy, if the HP cylinder cutoff is unchanged, it will absorb a proportionately larger share of the smaller potential power leaving the following stages less energy to extract. A really well designed compound is going to need independent cutoff on each stage that adjusts on the fly according to whatever steam temperature and pressure is entering the engine (and, if you really want to do it right, according to the condenser pressure as well). This is starting to involve a LOT of complication; personally, I'd rather design a variable valve timing system for an ICE, that can at least be handled with a single cam phaser.

Now, let's assume you want to manufacture these suckers. The compound is going to be more expensive, period. OK, I've had people argue this point with me but, in all cases, they had no practical experience in engine manufacturing. Remember that the compound is bigger? Well, there's some added materials expense right off the bat although I consider this the lesser of the costs. A multicylinder simple expansion engine uses identical parts in each cylinder and this gives a cost advantage the compound can't overcome. Every distinct part you build is going to require some sort of tooling, and that costs money. If you build 2 or 3 cylinder sizes, that is going to require much more tooling expense than for a simple engine. However, that isn't the end of it. Let's take pistons, for example. In a 3 stage compound we are going to need one of each size piston whereas the simple will need three pistons of equal diameter; it's hardly a surprise that the cost per part drops as the production quantity goes up. You have more parts across which to amortize the tooling costs. Same for the setup expenses. Longer production runs are generally more efficient. And so on.

I think I left a couple small things out, but that's why I think compound piston engines pose a liability.

Regards,

Ken



Edited 1 time(s). Last edit at 02/29/2016 05:30PM by frustrated.
Re: to replace a diesel
February 29, 2016 08:41PM
It makes sense, especially since for my application loosing a few percentage points of efficiency isn't a big deal like it might be where fuel cost is a factor. The whole idea is running on very cheap or free fuel anyway.

Now I have a little question, which no one has an answer for most likely because there are too many variables. How much speed can I get out of a double acting engine with small flywheel, if the components were reduced in weight as much as possible, aluminum piston, hollow piston rod, aluminum I beam style connecting rod. IC engines deal with vibration by having the same weight opposing itself. 4 cy has two going up and two down at the same time. The little inline 3 cy engines have a balancing shaft internally. Perhaps 2 cyl with 180 dg crank and electric starter motor, belt driven starter/generator like certain old small IC engines had. Single cyl IC engines can go pretty fast, but they have short strokes and all aluminum piston and rod. If I could get as much as 15-1600 working RPM out of one it would be sufficient, might work at 3/4 the speed of the diesel but it would work.

And if a double acting engine won't do it, a single acting engine would, but take double the cyl area to get the same torque.

Got a roof on my storage shed, and put some maple in it to dry for experimenting with block bearings like those [woodexbearing.com] had to cut a hard maple bout a ft through where I set up the sawmill last week, so it was the first to get sawed, just squared em and then halved them so they would dry without splitting too much. Get my shop floor space relieved from being storage space and I can start doing something.
Re: to replace a diesel
March 02, 2016 12:00AM
Thanks For the nice write Ken. Will take a few reads to fully digest.

Delta P should be a factor in blowby, compounding allowing for a lower delta plus catching anything blown by. A model I have showing the profile of the single has significant pressure at TDC where it is at a disadvantage to create work. Good dyno runs should yield good figures to apply polytropic variables to build a better model.

The Kent's Mechanical Engineers book's seem to associated higher efficiencies with the compounds also. That's a concern of mine.

Indeed we need to apply DFM principles et al, steady state vs reversable, on demand power vs economy etc. I'm not convinced I have, or have seen the "ideal" yet. Anyway, still trying to introduce rubber to road.

Sorry to deflect your Topic a bit ReubenT, not exactly how to replace a diesel, but certainly steam. -Keith
Re: to replace a diesel
March 02, 2016 06:45AM
Hi Keith,

People have been quoting Marks at me for years and I get more skeptical each time. My problem isn't with Marks, I assume their data is correct. My problem is that I think it's an apples and oranges comparison. Marks generally talks about heavy, lower pressure, lower temper stationary engines running with water cooled condensers .... almost the diametric opposite of vehicular plants. I keep imagining trying to derive rules for a turbo-diesel automobile derived from data taken from a Wärtsilä container ship engine; yes, they both run on the same cycle but the scale and applications are so far apart that the practical implementation is going to vary drastically.

Actually, I believe the same thing about Stumpf. For years people have been telling me that uniflow is the best way to go based on his work. As it is, I have no issues with what Stumpf said. The problem is that, again, he was talking about lower steam conditions combined with much bigger engines and far better condensing. Some of Stumpf's losses attributed to heat transfer between the steam and cylinder wall is going to disappear when you go to highly superheated steam with its reduced conductivity. The condenser is also a HUGE variable. Those water cooled stationary condensers can put out vacuums of 20" Hg and even better -- you're not going to get close in an automobile because air cooling is less effective and the size constraints also limits your results. The higher exhaust pressure equates to higher recompression figures that have to be offset by increasing the clearance volume. Unfortunately, increasing clearance volume offsets the efficiency gains we wanted from uniflow. I believe the answer is a semi-uniflow. Exhaust the majority of the steam out the bottom of the cylinder to keep most of the benefit from Stumpf's rules but have an auxiliary exhaust poppet valve in the cylinder head that vents off the higher recompression steam pressure through most of the upwards piston stroke so as to minimize clearance. Close the auxiliary valve at, say, 30% BDC and recompress to about admission to further minimize the effects of clearance, again as per Stumpf. The fact that you aren't tying up a bunch of horsepower in recompression is pure gravy, the engine volume can drop something like 30 to 40%. This isn't just me, the Scott-Newcomb was one of the more advanced designs of its time and it employed semi-uniflow, as did the Prof. Staley car at Purdue. I'm reasonably sure the French-Coats and Endurance cars did likewise.

Like I said, it isn't that Stumpf is wrong but rather that his principles are being applied without adequate regard for unique operating conditions. I also have the same reservations about Marks.

Regards,

Ken
Re: to replace a diesel
March 02, 2016 07:14AM
Thanks for the follow up Ken. At least it confirms we're on the same page of the same book. I liked your ORC write up in the bulletin too BTW. -Keith
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