Why is the Stirling Engine Not More Widely Used?

[jimmydasaint]

I am a bit baffled why the Stirling Engine is not more commonly used when we, as a global community, need to turn out engines which are quiet and clean with a lack of heavy pollutants? What are the restrictions stopping the use of this beautiful engine?

 

The modern Stirling Engine is a clean and efficient engine. This is because the heat driving the pistons is supplied from outside the engine and transferred through heat exchangers to the piston volumes. Thus the creation of pollutants such as NOx can be avoided. The external combustion aspect enables a Stirling Engine to operate equally well on multiple types of fuel, such as natural gas, propane, gasoline, diesel, ethanol85, bio-diesel or even heat from the sun. The extra quiet operation of the Stirling Engine is one of the engines best features. Many Stirling engine configurations are balanced by the nature of their construction, and since the fuel is burned slowly and constantly outside the engine, there are no explosions to muffle.

 

A Stirling cycle is truly reversable (this means that if you heat and cool the heat exchangers of the engine you get power out or if you power the engine you get heating or cooling out), and many engines can be used as a heat pump when driven by a motor or even another Stirling engine

 

Link

 

http://upload.wikimedia.org/wikipedia/commons/c/cc/Alpha_Stirling.gif

 

 

Edited February 15, 2012 by jimmydasaint

[Schrödinger's hat]

Mostly power to weight, they're impracticle. I don't know how efficient the best ones are, but it is also probably an issue.

[jimmydasaint]

Mostly power to weight, they're impractical. I don't know how efficient the best ones are, but it is also probably an issue.

 

Having said that, they can tick off 2 boxes out of three for me - lack of serious noise or gas pollution. The efficency issue is one of mechanical design surely?

 

Something extra to support these engines:

 

A Stirling engine uses the Stirling cycle,­ which is unlike the cycles used in internal-combustion engines.

 

•The gasses used inside a Stirling engine never leave the engine. There are no exhaust valves that vent high-pressure gasses, as in a gasoline or diesel engine, and there are no explosions taking place. Because of this, Stirling engines are very quiet.

•The Stirling cycle uses an external heat source, which could be anything from gasoline to solar energy to the heat produced by decaying plants. No combustion takes place inside the cylinders of the engine

 

Link to Article

 

Edited February 15, 2012 by jimmydasaint

[Schrödinger's hat]

Having said that, they can tick off 2 boxes out of three for me - lack of serious noise or gas pollution. The efficency issue is one of mechanical design surely?

 

The smaller the temperature difference between the hot and cold parts, the lower the efficiency.

Requiring all your heat be conducted puts a bit of a damper on getting a high temperature in the hot section.

It also limits the rate of energy throughput.

Any heat source hot enough to make a decent amount of energy (enough to compete with a combustion engine) will be useful to power a steam turbine, or make a good fuel for a combustion engine.

There is some precedent for their use in solar collectors, but again I gather that some kind of turbine is a better option.

[jimmydasaint]

The smaller the temperature difference between the hot and cold parts, the lower the efficiency.

Requiring all your heat be conducted puts a bit of a damper on getting a high temperature in the hot section.

It also limits the rate of energy throughput.

Any heat source hot enough to make a decent amount of energy (enough to compete with a combustion engine) will be useful to power a steam turbine, or make a good fuel for a combustion engine.

There is some precedent for their use in solar collectors, but again I gather that some kind of turbine is a better option.

 

Thanks for the information. What a shame that we cannot use these more commonly. The following site comes to the same conclusions:

There are a couple of key characteristics that make Stirling engines impractical for use in many applications, including in most cars and trucks.

 

Because the heat source is external, it takes a little while for the engine to respond to changes in the amount of heat being applied to the cylinder -- it takes time for the heat to be conducted through the cylinder walls and into the gas inside the engine. This means that:

 

•The engine requires some time to warm up before it can produce useful power.

•The engine can not change its power output quickly.

These shortcomings all but guarantee that it won't replace the internal-combustion engine in cars. However, a Stirling-engine-powered hybrid car might be feasible.

 

HowStuffWorks

[chilehed]

In a size that would move a car, they're REALLY inefficient. There are serious materials issues because the heat flux has to pass through the metal of the engine to get to the working fluid, which limits the working temperature and thus thermal efficiency. They have a low specific output, they take a long time to get started, and are extremely slow to respond to changes in power demand.

 

There's a real difficulty with sealing the working fluid within the engine. The best efficiency is with helium, but it leaks out over time so you need a bottle of the stuff hooked into the system, which is expensive. You could use air 'cause it's free, but it's less efficient and there's an explosion hazard due to the combination of high temperature, oxygen and lubricating oil.

 

It's got steampunk appeal, but it's not really a terribly practical engine. If it were then we'd be using it.

Edited March 3, 2012 by chilehed

[InigoMontoya]

Almost as an aside ('cause I'm not arguing anything else chilehed said) but... Nitrogen is damned near as cheap as air and eliminates the stated explosion hazard.

[chilehed]

Almost as an aside ('cause I'm not arguing anything else chilehed said) but... Nitrogen is damned near as cheap as air and eliminates the stated explosion hazard.

You really think that maintaining a nitrogen working fluid is as cheap as air?? You need bottles to store it, plumbing to connect it, a control system to operate it, more space to package it, higher maintenance costs to keep it operating, it adds a lot of mass....

 

The added costs are significant. It's a deal breaker.

Edited March 3, 2012 by chilehed

[InigoMontoya]

Well, it's not like you have to carry around a Nitrogen generator or a K bottle with you. You just need to have access to it so that you can refill in the event you system leaks. Shouldn't be any worse than recharging the air conditioner on your car right now. You've got a schrader valve coming out of your engine block. When you get your oil changed(*) for an extra $5 the guy checks the pressure and tops you off. Alternatively, they make small disposable gas bottles that can be filled with just about anything. The consumer is "used to" CO2 for paintball guns but they can be ordered with other gases in them. If such an engine were in mass production (it won't be for the other reasons listed), I imagine you'd be able to pick 'em up for a few bucks at any auto parts store. True such a bottle may not FILL your engine, but then, one quart of oil doesn't FILL the sump on your current vehicle. It just tops it off... Which is all that you'll need in the vast majority of situations.

 

Pick your size (there are more, but you get the idea)...

 

 

 

(*) Oil change probably not required for a Sterling. The point being "when you have some routine maintenance done."

Edited March 4, 2012 by InigoMontoya

[Enthalpy]

The Stirling engine has reciprocating pistons, hence it is big, heavy, and it vibrates. Turbines are better in all these aspects.

 

The only advantage of a Stirling over any other design is its heat exchanger which works on one single side of the solid hence can use a fine powder with a big exchange area with the gas. This improves operation when heat is available at a moderate temperature.

 

But design a good (=efficient and compact) heat exchanger and any engine will be as good as the Stirling. It's less easy because you exchange heat between two gases at different pressure, hence need tightness. Easier if your fluids are clean, for instance a closed-loop gas turbine heated by Sunlight.

 

Anyway, the Stirling still exists just because it's a bad habit from renewable energy guys.

Edited May 13, 2012 by Enthalpy

[Elite Engineer]

In the late 1970's and early 1980's, NASA and the Department of Energy teamed up to make an alternative automotive engine due to the
nation's oil dependcy scare in 1978. It was called the MOD1 and was quite successful in operation, however funding was cut in the mid 80's and the project never reached it's final phase. Here's an article from "Popular Science", from Jan. 1983 giving a very in-depth look into the application and research of stirling engines for automotive use.

 

http://books.google.com/books?id=RN4_jLbVO3YC&pg=PA50&dq=Stirling+auto+engine+a+lot+of+progress,+but+Spirit#v=onepage&q&f=false

[johnhenryford]

Nice article and good pictures of the Mod I engine, but a little newsflash - the Mod II was completed by 1986 and overcame most of the limitations people still think exist with Stirling engines. It used Hydrogen as the working gas (small amount, so little safety hazard) and could adjust the pressure in the engine to meet the dynamic power needs for an automobile. It could run on any fuel since it was external combustion and had no need for a muffler or catalytic converter since noise and emissions were so low.

 

They tested it in a 1985 Celebrity and compared to the 4 cyl. internal combustion engine it had higher torque and slightly lower horsepower, could reach 0-60 mph faster by 0.6 seconds, and got better gas mileage by 10 mpg (combined score) with 33/58 miles per gallon for city/highway driving. In the report by NASA they claim it could be manufactured cheaply enough to compete with internal combustion engines.

 

The NASA report:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880002196_1988002196.pdf

or here if the first link doesn't work:

http://www.mechanicalengineering.net/publications/automotive-stirling-engine.pdf

 

Interesting old video produced by NASA on the same subject:

 

On the other hand, it was a pretty complicated engine and might be easier now to use a Stirling powered generator for an electric hybrid car.

[overtone]

Even relatively crude Stirling engines are far more efficient than any internal combustion gas engine in steady operation. They have control and efficiency problems as stand alone car engines, where speed must be varied precisely and over a wide range, but the most obvious reason they have not been jumped on for use in hybrids is probably the qwerty phenomenon - they'd be ideal for feeding power to an electric motor and charging a battery with the leftover, in normal use could be left to run in a parking lot charging a communter battery without much problem, all kinds of advantages.

 

They operate best at high temps (the larger the temp gradient, the more efficient the engine) and although few parts move the ones that do are hard to lubricate, so they require expensive materials - that's a pretty big issue in a modern manufacturing plant.

 

They are the most efficient developed conversion mechanism for solar electricity, but in current designs the entire engine must be mounted with the collection dish and moved with it to track the sun - an expensive setup with lots of moving parts under load.

 

Most people who look into them end up advocating more effort their way - at least as much as has been put into the still distant prospect of fusion power, say.

[Junkyardnut]

I am not sure if you are aware of a company named SES which is short for Stirling Energy Systems (http://www.greentechmedia.com/articles/read/stirling-energy-systems-dish-engine-solar-maker-files-for-chapter-7-bankrup It was the most promising business about more stirling engines but it went kaput! The company was passed over for photovoltaics in a huge project . Maybe too many moving parts to keep up with maintenance. You can still buy them and use them to run your room air conditioners if you care to..

Also, click http://www.infiniacorp.com/en/ They are probably the next generation on stirling engines.... check it out!

Edited March 29, 2013 by Junkyardnut

[Elite Engineer]

I think all the stirling engine (for car applications) needs is more research, and general knowledge of how it works so it would be more attractive to investors and the public. The MOD I and MOD II projects only had a couple of years to be developed before funding was cut, so of course they're not going to be perfect machines. I'm certain if we hadn't added the improvements (radiator, torque converter, etc) to the modern ICE it wouldn't be used as efficient as it is (you call 33% efficient..heh!). The ICE engine had roughly 100 years to be tweaked and developed. I believe the main issue is, the stirling isn't mainstream or understood enough by the public, sorta similar to hybrids.

 

Its no secret that the stirling is more efficient (with a decent temp. differential) than a standard ICE. I hope to see these in generator applications soon.

[DiegoTom]

"Phase I Results from the Stirling Powered Vehicle Project funded by NASA

 

 

Ten months of operation were with Air Force personnel at Langley Air Force Base. VA, where over 1100 hr and 4000 mi were logged on the Langley flight line. The Stirling-powered van was operated on unleaded gasoline. JP-4 aircraft fuel, and diesel fuel at Langley Air Force Base. Two months of operation were completed with Deere & Company personnel in the Moline. IL area where over 175 hr and 2650 mi were logged on a Deere mail delivery route.

The Air Force provided a 1986 General Motors (GM) multistop (delivery) van (Fig. 1) powered by a standard 6.2-L (379-in3) V-8, diesel engine (145 hp (108.8 kW) at 3600 rpm). Curb weight of this vehicle is 6800 lb (2677 kg), with a maximum gross vehicle weight (GVW) of 8600 lb (3386 kg). Engine specifications and a power and torque curve are shown in Table 1 and Fig. 2. When the GM multistop van arrived at MTI, it was driven approximately 1100 mi to break in the drivetrain to ensure that the vehicle was sound. After the vehicle break-in was completed, the diesel engine was removed and a Mod I Stirling engine mockup was installed to determine what modifications would be required. The only major alteration required was a cutout in the front crossmember. This modification was needed so that the engine and transmission could be properly installed and aligned.

Oil samples were taken 10 times during the one-year evaluation period and sent to Deere & Company for detailed analysis. No degradation of the oil was noted and Air Force operators and maintenance personnel observed that the oil appeared to be in "like new" or "virgin" condition.

Fuel economv for the Stirling- powered van was reported bv the Air Force to be 6.3 mpg with all fuels used during the 10-month evaluation period. Fuel economy numbers include fuel usage for both the gas-fired, front window defogger and the rear-compartment heating system. This compares with an average of 4.3 mpg for four gasoline-powered vans and 8.1 mpg for nine diesel-powered vans used at Langley in similar missions during the same period.

The 75-hp automotive Stirling engine, although underpowered for the Air Force multistop, has demonstrated that it has the potential to be a viable multifuel engine for lightduty trucks and vans of the future.

As of January 1988. seven experimental. first-generation. Mod I automotive Stirling engines CASE'S) were built and operated in test cells and vehicles for over 18 000 hr."



http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880018596.pdf

[Enthalpy]

Then the true strength of the Stirling is to choose a really bad competitor.

 

Because my Citroën Cx is also from 1987, has also a (turbo)-Diesel with also nearly 150hp, but it needs only a 2.4L 4-cyl to achieve the power instead of a 6.2L V8, and at 160km/h (100mph) it consumes 6L/100km or 0.026gal/mile or 39mpg, not 6.3mpg.

[Moreno]

Well, this is unlikely, of course, but I think that in distant future portable Stirling engines may find wider use (for example in vehicles) if new ways to use nuclear power will be invented. A portable Stirling engine is one of the best ways to convert energy of a portable fusion reactor...

Edited October 12, 2016 by Moreno

[Elite Engineer]

Well, this is unlikely, of course, but I think that in distant future portable Stirling engines may find wider use (for example in vehicles) if new ways to use nuclear power will be invented. A portable Stirling engine is one of the best ways to convert energy of a portable fusion reactor...

I once saw a video where Bill Nye explained solar powered stirling engines will have more application in space (space stations, colonies, transport, etc.), due to their extremely simple design, and their high efficiency of running off heat.

[FouRockNET]

I am not an expert but this is what I understand after researching this on and off for quite some time (10 years):

 

• Stirling engines are not great for uses in cars and other applications where you want to regulate the output on demand.
• The nature of the engine is such, that in practice it will probably lose a great deal of energy with heat loss, heat transfer etc.
• Most of the engines require maintenance because of the inner atmosphere (the gas used for transfer).

• It is and was used rather to supply a constant output with a regulated heat input. That is why it is suitable for solar applications and waste heat generators, for which it has been used (on a small scale) for some time.
• It is not used in large applications, because apparently you get better efficiency with CCGT (combined cycle gas turbine).
• Generally, I've seen engineers use steam turbines where one would imagine a stirling engine. I have no idea why, but I imagine they have a good reason for that (cost, maintenance, input and output regulation etc.).

 

Due to the nature of the engine, I have allways dreamed of a stirling powered by biomass and this year I have found two projects where this is done:

http://www.okofen-e.com/de/pellematic_smart_e/

http://www.microgen-engine.com/buy-engage/

 

Both use a sterling engine from the same company. One costs around 23000 Euro, the other 11000 Euro. They use waste heat from the fireplace. One has a buffer tank which is used to store warm water, runs on pellets and is automated. The other is just a fireplace. The advantage is that the heat that is not used by the engine is used to heat the house and water.

 

I am not sure how well these will work in the long run and what the maintenance costs will be, but they bassically represent what a sterling engine is suitable for (collecting waste heat and converting it into energy).