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Re: Modern Steam - Formula S Racecar May 03, 2021 09:43PM |
I'll agree that a steam engine needs higher rpm to be competitive -- despite assertions to the contrary, steam engines are not THAT different than internal combustion. Both engines obey the same basic mechanical rules and horsepower is going to be a product of torque and rpm. Torque is always going to be limited if reasonable efficiency is desired due to the need to reduce MEP. All this said, traditional direct drive is a losing proposition. Since horsepower is going to rise with rpm, a direct drive car won't reach peak power until reaching peak speed on the track. By contrast, gearboxes allow the engine to be spun up to peak power multiple times during an acceleration run, achieving higher average output. See attached drawing showing the conceptual differences between direct drive, a four speed transmission, and continuously variable transmission. Obviously, this is a simplified example and not representative of any actual system.
Feed water heaters are a non-issue in condensing cars. Condensers and feed heaters are both heat exchangers, each of them working with steam coming out of the engine, stacking heat exchangers to deal with the same working fluid should set off a few alarm bells. Steam leaving a cylinder is going to be saturated, in any realistic steam car design, since it would be wildly wasteful to have any but a small amount of superheat at release.. Assuming exhaust at atmospheric pressure (which is pretty likely given the limitations of air cooled condensers), the steam is going to enter the condenser at 212 degrees F. Operators of large, serious steam plants work to minimize condensate depression -- this is the temperature the condensate is cooled below saturation. Every fraction of a degree water is cooled below condensation temperature represents a loss of heat to the environment. (And a bigger, heavier radiator producing more aerodynamic drag.) A small amount of subcooling is necessary to prevent flashing in the condensate pumps and the condenser hotwell, especially if pressure changes rapidly -- but only a small condensate depression is maintained. This means that an efficient steam plant will be pumping feed water into the boiler at almost the same temperature as steam leaving the engine. Since heat exchangers require temperature differentials to operate, and since their effectiveness is proportional to the difference in temperatures, it is simple to see that feed water heaters can contribute little to a properly designed condensing system.
Likewise, radiant burners appear to possess little to no advantages. Arguing their superiority based on their use in cooking seems a false comparison. First of all, cookware is not a well designed heat exchanger. A pot or pan has a large surface against which a gas flame can impinge. This large surface allows a gasses to "pile up" before the surface without making contact -- we can get a booger of a boundry layer. Obviously, radiant heat is not affected by a boundary layer. This is not the situation with a boiler which is comprised of multiple tubes having small passages between them, the gas flow is broken up and forced into intimate contact with the tubes. Beyond this, there is the matter of temperatures. Let's say we want our race car to operate with steam at 1,000 degrees F. Radiant heat flows from hotter to cooler sources, just like convection or conductive heat transfer. This is a lot hotter than we cook some bacon. This means that the radiant heater has to be well above 1,000 ... let's say 1,500 in order to support a high rate of transfer. The radiant heater is bought up to temperature by combustion gasses and, since heat goes from hotter to cooler, these gasses must be hotter than the radiant emitter ... let's say the burner runs at 3,500 degrees with the gasses leaving the burner running about 1,800 degrees. Now that's a lot of hot gas, and the only way to avoid wasting that heat is to pass the gasses past cooler boiler tubes -- which means we are talking about convective heating. (Since radiant heating is line of sight, only the tubes neares the burner receive infrared, the others are convectively heated.)
Now, here's the rub. Conventional burners put out a LOT of radiant energy. Since the radiation can only heat the tubes in line of sight, the convective heat will by necessity largely heat the tubes further from the burner. in fact, the tubes closest to the burner often receive little but radiant heat because the tube temperature is approaching the temperature of the gas flowing past. Heck, it is possible for the backside of the radiantly heated tubes to emit infrared heat which will be absorbed by the nearest cooler tubes. Anyhow, from a practical standpoint, the radiant burner does almost exactly the same thing as a conventional burner, it pumps infrared radiation into the nearest tubes and hot gasses heat the tubes that follow. The only practical difference appears to be greater complexity.
Lest this seem like speculation, remember that steam car boilers run from around 75 to 90 percent efficient...with Abner Doble reporting 93 percent in one instance, but I am sure he fudged a bit by measuring efficiency while using room temperature feed water. In any case, at 90 percent efficiency, you are approaching the practical limit for a compact boiler -- once again because heat flows from hot to cold. As the cooling gasses interact with cooler tubes, the rate of heat transfer diminishes as the temperature differential drops. Improving efficiency much over 90 percent requires an impractically large boiler. This begs the question: "if some boilers are already about as efficient as is possible, what's the advantage of changing burners?" Honestly, I see no advantage.
We also need to avoid the all-too-common trap of trying to wring every last BTU out of the steam by expanding as much as possible. I keep hearing arguments that piston engines can achieve the same large expansion ratios as turbines but it's easy to argue this is because the proponents don't fully understand some critical differences between piston expanders and properly designed turboexpanders. (A properly designed turbine stage causes a non-turbulent reversal in steam flow. Machines designed to be carried along by the steam flow suffer significant reductions in efficiency -- one example is the Tesla turbine. Likewise, the pressure drop between turbine stages converts pressure to velocity which the turbine rotor is designed to exploit, unlike piston engines.) One should be cautious trying to achieve lower MEP than internal combustion engines, no matter how many steam fans advocate very short cutoff. From the standpoint of raw theory, they are absolutely right, but practical application is being ignored. A few years ago, Art Gardiner (PhD in Mechanical Engineering) gave a lecture in which he discussed the concept of FMEP -- Friction Mean Effective Pressure. This is the MEP needed to overcome friction and it is pretty constant for any given engine speed. At some point, it is quite possible to expand steam to the point where power developed in the latter part of the stroke is less than the friction produced by the engine during that same portion of the stroke. This can be especially true for compound engines, given that the lower pressure pistons not only operate at lower MEP, they also have greater friction due to the greater piston and piston ring diameter and other loads due to heavier piston mass. This ignores the loss of power occassioned by the interstage pressure drop -- which is inherent to compound engines because you need a pressure differential to get steam to flow from one cylinder to another.
Anyhow, that's my two cents.
Edited 2 time(s). Last edit at 05/03/2021 09:56PM by frustrated.
Feed water heaters are a non-issue in condensing cars. Condensers and feed heaters are both heat exchangers, each of them working with steam coming out of the engine, stacking heat exchangers to deal with the same working fluid should set off a few alarm bells. Steam leaving a cylinder is going to be saturated, in any realistic steam car design, since it would be wildly wasteful to have any but a small amount of superheat at release.. Assuming exhaust at atmospheric pressure (which is pretty likely given the limitations of air cooled condensers), the steam is going to enter the condenser at 212 degrees F. Operators of large, serious steam plants work to minimize condensate depression -- this is the temperature the condensate is cooled below saturation. Every fraction of a degree water is cooled below condensation temperature represents a loss of heat to the environment. (And a bigger, heavier radiator producing more aerodynamic drag.) A small amount of subcooling is necessary to prevent flashing in the condensate pumps and the condenser hotwell, especially if pressure changes rapidly -- but only a small condensate depression is maintained. This means that an efficient steam plant will be pumping feed water into the boiler at almost the same temperature as steam leaving the engine. Since heat exchangers require temperature differentials to operate, and since their effectiveness is proportional to the difference in temperatures, it is simple to see that feed water heaters can contribute little to a properly designed condensing system.
Likewise, radiant burners appear to possess little to no advantages. Arguing their superiority based on their use in cooking seems a false comparison. First of all, cookware is not a well designed heat exchanger. A pot or pan has a large surface against which a gas flame can impinge. This large surface allows a gasses to "pile up" before the surface without making contact -- we can get a booger of a boundry layer. Obviously, radiant heat is not affected by a boundary layer. This is not the situation with a boiler which is comprised of multiple tubes having small passages between them, the gas flow is broken up and forced into intimate contact with the tubes. Beyond this, there is the matter of temperatures. Let's say we want our race car to operate with steam at 1,000 degrees F. Radiant heat flows from hotter to cooler sources, just like convection or conductive heat transfer. This is a lot hotter than we cook some bacon. This means that the radiant heater has to be well above 1,000 ... let's say 1,500 in order to support a high rate of transfer. The radiant heater is bought up to temperature by combustion gasses and, since heat goes from hotter to cooler, these gasses must be hotter than the radiant emitter ... let's say the burner runs at 3,500 degrees with the gasses leaving the burner running about 1,800 degrees. Now that's a lot of hot gas, and the only way to avoid wasting that heat is to pass the gasses past cooler boiler tubes -- which means we are talking about convective heating. (Since radiant heating is line of sight, only the tubes neares the burner receive infrared, the others are convectively heated.)
Now, here's the rub. Conventional burners put out a LOT of radiant energy. Since the radiation can only heat the tubes in line of sight, the convective heat will by necessity largely heat the tubes further from the burner. in fact, the tubes closest to the burner often receive little but radiant heat because the tube temperature is approaching the temperature of the gas flowing past. Heck, it is possible for the backside of the radiantly heated tubes to emit infrared heat which will be absorbed by the nearest cooler tubes. Anyhow, from a practical standpoint, the radiant burner does almost exactly the same thing as a conventional burner, it pumps infrared radiation into the nearest tubes and hot gasses heat the tubes that follow. The only practical difference appears to be greater complexity.
Lest this seem like speculation, remember that steam car boilers run from around 75 to 90 percent efficient...with Abner Doble reporting 93 percent in one instance, but I am sure he fudged a bit by measuring efficiency while using room temperature feed water. In any case, at 90 percent efficiency, you are approaching the practical limit for a compact boiler -- once again because heat flows from hot to cold. As the cooling gasses interact with cooler tubes, the rate of heat transfer diminishes as the temperature differential drops. Improving efficiency much over 90 percent requires an impractically large boiler. This begs the question: "if some boilers are already about as efficient as is possible, what's the advantage of changing burners?" Honestly, I see no advantage.
We also need to avoid the all-too-common trap of trying to wring every last BTU out of the steam by expanding as much as possible. I keep hearing arguments that piston engines can achieve the same large expansion ratios as turbines but it's easy to argue this is because the proponents don't fully understand some critical differences between piston expanders and properly designed turboexpanders. (A properly designed turbine stage causes a non-turbulent reversal in steam flow. Machines designed to be carried along by the steam flow suffer significant reductions in efficiency -- one example is the Tesla turbine. Likewise, the pressure drop between turbine stages converts pressure to velocity which the turbine rotor is designed to exploit, unlike piston engines.) One should be cautious trying to achieve lower MEP than internal combustion engines, no matter how many steam fans advocate very short cutoff. From the standpoint of raw theory, they are absolutely right, but practical application is being ignored. A few years ago, Art Gardiner (PhD in Mechanical Engineering) gave a lecture in which he discussed the concept of FMEP -- Friction Mean Effective Pressure. This is the MEP needed to overcome friction and it is pretty constant for any given engine speed. At some point, it is quite possible to expand steam to the point where power developed in the latter part of the stroke is less than the friction produced by the engine during that same portion of the stroke. This can be especially true for compound engines, given that the lower pressure pistons not only operate at lower MEP, they also have greater friction due to the greater piston and piston ring diameter and other loads due to heavier piston mass. This ignores the loss of power occassioned by the interstage pressure drop -- which is inherent to compound engines because you need a pressure differential to get steam to flow from one cylinder to another.
Anyhow, that's my two cents.
Edited 2 time(s). Last edit at 05/03/2021 09:56PM by frustrated.
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Re: Modern Steam - Formula S Racecar February 04, 2022 01:43PM |
Rick, I plan on putting all information up on the forum and hope for a lot of good information and critique. I very much doubt that anything we will come up with would be patentable or worth trying to patent and would rather have any insights and plans available for the club even if it turns out to be what not to do. 
Expect more information and drawings on the steam generator thread sometime tonight or tomorrow.
SteveW
Expect more information and drawings on the steam generator thread sometime tonight or tomorrow.
SteveW
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