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The Steam Powered Racing Model Hydroplane
This branch of boat modelling is I unique to the UK, it demands more from the builder/designer than any other form of model making I am aware of, which may account for the fact that its exponents are few and far between. It will be a very sad day if we lose it. These boats are the extreme of small steam engine performance where as mentioned earlier Bob Kirtleys PiscesII turns out 385 BHP per litre of cylinder capacity, 5 HP from 13 cc. This is a formidable performance and anyone can be forgiven for disbelieving the accuracy of this statement; consider the facts. There are two classes of Hydro. in this category, class A, up to 9lbs weight, 15cc IC engine or steam; and class B, up to 16 lbs weight, 30cc IC engine or steam. Notice that apart from the fact that the steamers are allowed to weigh a bit more than the IC engined boats they compete on equal terms and their speeds are very close. In class B Bob Kirtleys boat is making 120 mph (200 kph) and the IC boats are at about 130 mph (235 kph). Now the 30cc racing two stroke engine turns out about 5, some say 7 BHP so it must be true that Bob's Steamer is making about the same BHP but it is of only 13 cc displacement which works out at 385 BHP per litre. I have been asked by persons corresponding through this web site; how is this level of power achieved? "From a blooming steam engine," as one man once colourfully put it!
Compression Ratio and Break Mean Effective pressure
In a spark ignition engine the compression ratio is up to about 9:1 fueled on petrol and maybe 12:1 fueled on Methanol, a glow motor is also about 12:1. A diesel operates at between 16 and 20:1and can be fueled on almost any light mineral or biological oil making exeptions for acidic oils and kerosene which is a very poor lubricant and usually spoils the injector pump!
Its what goes on in the cylinder that matters and how many bangs you get per minute. Let me first compare the glow motor and the single cylinder uniflow Steam engine. Obviously the comparison of steam and IC hydroplane is the only one that is available to me. Their running speeds are generaly comparable at about 15,000 revolutions per minute so that can at once be largely eliminated as a factor. So it really is all down to what goes on in the cylinder. Break mean effective pressure (BMEP) is a measured and or calculated average pressure inside the cylinder of any piston engine, during the power stroke whilst it turns a controlled load, usually measuring power output. These pressures can be remarkably low, for example those tiny Cox glow motors may scream for attention but their BMEP is only about 25 Lbs per square inch (about 2 KG per square CM). Whilst the pressure is relativly low the speed is encouraged to rise and rise to levels unheard of 50 years ago when I did my aeromodelling apprentiship. The tiny 0.1 and 0.2cc Cox motors do 30,000+ rpm delivering 30,000 power strokes per minute which despite the low average cylinder pressures still represents lots of power for so small an engine. Bob Kirtley's steam engine has the big advantage that its flash steam generator develops continuous steam pressures that only a modern diesel can even approach. A normal petrol engine of modern design produces a BMEP of about 650 PSI. In the IC engine the burning gas temperature and pressure is at its maximum very soon after or even at Top Dead Centre and thereafter as burning is completed what is left is expansion, cooling and further pressure drop. In a steam engine, steam is continually admitted well into the power stroke, adding more heat as expansion proceeds. In the case of Bob Kirtleys engine steam pressure may be as high as 2000 PSI and cut off is at about 60% of the power stroke so the MEAN AVERAGE is raised well beyond that which may be achieved in the cylinder of any petrol or diesel engine.
This is done at the obvious expence of efficiency but that is not the issue in simple speed competition events. Pressure ratios and the resulting BMEP is the factor that gives the astonishing figures to be found in Uniflow steam engines working on the edge of their possibilities. Steam can cheat a bit because the heat can be fed in during the power stroke but compression ratio still matters. The higher the compression ratio the less 'space' has to be wasted just filling the cylinder as the inlet valve opens.
It is this continuous battle of extremes, Steam verses IC that is the fascination of these boats for me.
The IC combustion space
The burning process in the cylinder of any IC engine is far less than ideal, the cylinder is of a heat conductive metal, cooled continually by water or direct air flow and the combustion space is continually changing its shape and volume! This is bad for efficient burning, it is the partial combustion of the fuel under these bad conditions that causes the concoction of polluting gases and acidic compounds in the exhaust.
Diesels are better
Compression ignition wipes petrol engines off the board as far as efficiency goes because of the much higher compression ratios that are achieved. On injection, a few degrees before TDC, the combination of high pressure and high temperature (about 450 degrees C) causes the atomised fuel to detonate, it doesn't just burn, it detonates like TNT. The burning is as complete as it will ever be at TDC. Then that gas expands 20 volumes delivering power (but less and less of it) and cooling all the way down until the exhaust valve opens. Far better than a petrol engine's 9 volumes. It is this greater volumetric expansion which makes Diesel exhaust gas much cooler than petrol engine's and why diesel engine exhausts and exhaust maniflods generally last much longer too. However the BMEP of a diesel still cannot approach the continual supply of superheated high pressure steam into the cylinder delivered by a small racing steam plant.
Jet Engine compression ratios
Yes they have pressure ratios and they make even a diesel engine pale into insignificance and this pressure ratio is at the heart of the huge improvements that make modern aircraft so much more fuel efficient than jets just 15 years old. The RR Trent 900 at optimum altitude can produce a pressure ratio of nearly 40:1! What is interesting is at 30,000 feet the static pressure outside is very low to begin with about 260 millibars or 4 PSI in normal terms. So combustion pressure is say 160 psi exhausting through a hole several feet in diameter! That really is one hell of a breeze. My air line has a maximum pressure of about 160 psi and a hole about 2 mm in diameter. Just feeling the thrust it is a pound or two; think of the energy maintaining that pressure through a 1 metre diameter hole, it beggers belief! OK the turbine is in the way and it takes 60 or 70% of the exhaust's thrust! I know that but it puts the thing into a relationship that we can all see as quite awesome in scale. Static thrust 90,000 lbs = 40 imperial tons, per engine and there are four of them on the Airbus. Much of the energy derived from the turbines is converted into another pressure ratio in the bypass compressor but that is extending this part of my site too far, go to the RR website for all the details about this triumph of UK engineering.
I hope the above notes go some way to explaining in simple terms the special conditions that can enable a steam engine's specific power (BHP per Litre) to out-perform IC engines at the extremes of its performance; provided that fuel consumption is not an issue! I hope to collect a few more pictures of these boats in the not too distant future. (As at March 2007)
There are only two books that matter on this subject, you need both to get the best of the history from piston valves to piston actuated (bash) valves to the best poppet valve engines of today, written by individuals that actually "did it", the technical detail is very sound.
The earliest was "Flash Steam" by Edgar T Westbury, first published I do not know when as my copy is a repro. by Tee Publishing; a strange company who choose not to help the purchaser by politely acknowledging previous publishers!
Much later "Experimental Flash Steam" by Benson and Rayman my copy is from Argus Books,Dated 1973 I believe it was repeated by Tee or another publisher who may or may not mention previous pressings.
I suggest you go to the UK site of the Steam Car Club of Great Britain www.steamcar.net home page "Hydroplanes" where Rick Benson has some personal experiences of the steam hydro. Yet more to read and pictures to inspire.