Response, September 1, 2012 by John Cozby
to:
Uniflow vs. Multi-Cylinder Compound
In The Steam Automobile Bulletin, Volume 26, Number 5, September - October, 2012 there is an article on page 33 by Tom Kimmel titled “Uniflow vs. Multi-Cylinder Compound”. In the article, Tom refers to a conversation between myself and Ken Helmick at the Sacramento meet. The purpose of this paper is to try and clarify any misconceptions. It is hoped that this short discussion is helpful in comparing the uniflow engine to the advanced, multistage, re-heat Rankine cycle engine.
A distinction must be made between two very different types of compounding in steam engines. One type of compound engine has no additional heating in the receivers. This type of engine could be said to have cold receivers. This is correctly called a bad design and loss occurs. The other type of compound engine has heat added in the receivers. These are called re-heating receivers. This type of engine could be said to have hot receivers. This type of engine is a good design and efficiency gains result. The no heat receiver type of compound is pathetic. The re-heat type of compound can be exceptional. It is multiple re-heating between stages that makes steam power plants efficient. The type of compound engine that is under discussion must be identified before any valid conclusions can be reached. The Cozby design engine is of the multiple re-heat type.
In the Bulletin, Volume 25, Number 3, May - June, 2011 there is an article by Karl Petersen on page 23 about the 130 foot long yacht Arrow. The Arrow set the steam speed record in 1903 of 45.06 miles per hour. The Arrow was running in 1902 — 110 years ago. The Arrow was powered by “A quadruple expansion engine with re-heat between all stages was the ultimate workhorse at that time,”. That was “real work” by a quad engine. Tom Kimmel commented on the Arrow on page 8 of the same Bulletin. In Marks’ Mechanical Engineers” Handbook, Fourth Edition, on page 1201 records a quadruple-expansion engine tested in 1921. “Tests by Schmidt of a 150 hp quadruple-expansion engine using steam at 794 lb per sq in abs at 815F, with 28.6 in. vacuum, and with superheating in the receivers . . . , show a total steam consumption of 5.12 lb. per ihp-hr, a thermal efficiency of 31.1 percent,”. That is “real efficiency”; better efficiency than any uniflow of that time. Quad expansion engines of the re-heat type do real work and they are efficient.
A puzzle concerns Dr. Stumpf and his book, The Una-Flow Steam-Engine, 1922. Dr. Stumpf greatly disparages the compound, the triple, and the quadruple expansion steam engines, yet in his entire book Dr. Stumpf never once mentions re-heat steam engines. Re-heating is the best means of improving efficiency. Dr. Stumpf’s treatment of minimizing losses is good and important, but the best way to improve efficiency is good and important too. The quadruple re-heat engines were beating the best uniflows, but to Dr. Stumpf they did not seem to exist. The uniflow engine cannot be a re-heat engine. (This may be a clue.) Dr. Stumpf was a very good promoter and salesman for his own product, but he was not entirely objective or exhaustive.
The statement in the Bulletin article, “the uniflow drove them (quadruples) out of practical use about a century ago”, needs a little amplification. The reason for the success of the uniflow was not entirely for the reasons presented in the article. The single cylinder, single stage uniflow was both simple and cheap. A quadruple engine was more complex and expensive. A uniflow with a single steam line to the engine made the furnace-boiler simple and cheap. A quadruple with re-heating in each receiver was more complex and expensive. Oil and coal were abundant and cheap. The best engine efficiency was not as great an issue. Most stationary uniflow engines did not need to operate at high overload conditions. Air pollution was not considered an important factor in choosing an engine. The simple, cheap, and less efficient uniflow dominated the general market. Things have changed. An automotive uniflow of four or six cylinders renders the argument for a single cylinder uniflow engine moot. Engine efficiency and economy are of paramount concern. Our country is dependent on foreign oil and fuel is expensive.
In the Bulletin article the statement is made: “the ratio of expansion (in a uniflow) can thus be made as high as in a multi-cylinder compound engine.” (This sounds good and such may be true in theory, but not in best practice.) The article goes on to state: “In Fig. 81 is shown an indicator diagram taken from a uniflow engine and a combined indicator diagram from a four-cylinder compound engine having the same expansion ratio.” It is stated that the two engines have the same expansion ratio. It is possible to have a uniflow with a very early cut-off and a quadruple with very low cylinder ratios so that the two could have the same expansion ratios. If the two engines do have the same expansion ratio and the quadruple has “cold” receivers, the uniflow is the best choice. Marks’ Handbook states: “. . . the efficiency varies directly with the ratio of expansion,”. It is recognized that expansion ratio is of primary importance to engine efficiency and that very high ratios of expansion are desirable.
The actual ratios of expansion of two automotive uniflow engines follows. Dr. Stumpf’s automotive uniflow engine had cut-offs of 9%, 25%, and 80%. At 9% cut-off, and assuming a 10% exhaust lead, this engine had a ratio of expansion of 10 to 1. In Sacramento I asked Harry Schoell what ratio of expansion his cyclone engine got. Harry replied that it got a ratio of expansion of 32 to 1. That is very high for a single stage engine. Assuming about 10% exhaust lead, a 32 to 1 ratio of expansion would mean a cut-off at about 3% of stroke. The drawing of the uniflow engine in the Bulletin article shows quite a large exhaust lead of about 25%.
Roger Demler proposed a two stage, compound, single re-heat engine to the Energy Research and Development Administration. The Energy Research and Development Administration looked at Mr. Demler’s proposal and declared it to be an advanced Rankine cycle engine that could be developed into the automobile engine of the future. ERDA was right. [Mr. Demler’s actual design was for 1,250 °F initial steam temperature and 1,500 °F reheat. The ideal efficiency is over 70%. This is reported in both the S.A.E. paper 760342 and DOE/CS-0125 (ERDA-77-54)] If a similar compound engine with a single re-heat were to have an initial temperature of only 1,200 °F superheat and a re-heat of only 1,200 °F (as in the Cozby design) the first stage can have an 8 to 1 ratio of expansion without condensation, and the second stage can have an 8 to 1 ratio of expansion without condensation. The expansion ratio of such an engine would then be 8 x 8 or 64 to 1. This is twice the expansion ratio of the cyclone uniflow engine, and 6.4 times the expansion ratio of Dr. Stumpf’s una-flow automotive engine.
The Cozbys’ three stage (triple) , two re-heat engine design has an expansion ratio of 8 to 1 for the first stage; 8 to 1 for the second stage; and 8 to 1 for the third stage. The Cozby engine therefore has an expansion ratio of 8 x 8 x 8 or 512 to 1. This is 51.2 times as great as Dr. Stumpf’s una-flow engine, and 16 times as great as the cyclone uniflow engine. Without even going into the quadruple expansion engine, it is clear that any assumption that the uniflow and the advanced multi-cylinder compound engines with high ratios and re-heats have the same ratios of expansion is wrong.
The Bulletin article further states that the multi-cylinder compound is not good because: “This is mainly due to receiver losses in the compound engine.” This statement is true for compound engines that do not have receiver re-heating. The Cozby triple expansion engine has re-heating to 1,200 °F in the first receiver, and re-heating to 1,200 °F in the second receiver. By referring to our Material balance sheet it is seen that steam exhausted at 591 °F and re-heated to 1,200 °F in the first receiver, and steam then exhausted at 592 °F and re-heated to 1,200 °F in the second receiver does not constitute a loss in the receivers. There is a good efficiency gain from the heat added to the steam in the receiver/re-heater of re-heat engines. This is because of two reasons: (1) the range of realizable expansion is greatly extended, and (2) because the vapor which is re-heated does not experience the loss of the latent heat of vaporization. Modern efficient steam power plants have re-heats and four stages.
By using re-heating receivers in multi-cylinder compound engines the ratio of expansion can be very high. When the expansion ratio is very high the efficiency is also high. Multi-stage re-heat engines do not suffer as much as single stage engines when operating at overload capacity. The Cozby engine can operate at about 1,000% overload capacity. A Cozby engine putting out about 40 horsepower at 1/8 cut-off can quickly increase its power to about 400 high torque horsepower under heavy load. Even at such high power out-put the Cozby multi-stage engine still has about twice the expansion ratio of the cyclone uniflow engine at its 32 to 1 expansion ratio rating. When Dr. Stumpf’s una-flow automotive engine with 10% exhaust lead goes to 80% cut-off for high power his engine has virtually no expansion ratio. With no expansion ratio, the una-flow engine’s efficiency suffers badly and the boiler capacity and condenser capacity is soon exceeded.
