Stir-Lec, a plug-in hybrid electric car with a Stirling engine from 1968

March 9, 2024
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This 1968 Opel Kadett hybrid-drivetrain prototype was way ahead of it’s time. It’s main issue was that it was using off-the-shelf lead-acid batteries, and there was only enough room for a small pack (The car in the header pic is just a random photo of a stock German Opel Kadett)

As an electric car in 1968, it was slow and had short-range. As far as performance, you could have swapped-in a larger motor and given it more amps. So, what could we do to provide more amps? A modern Lithium battery pack could easily provide an adequate amount of amps for enough acceleration to equal the gasoline version. This prototype used 14 of the common 12V batteries, for a nominal voltage of 168V.

The top-speed was near 55-MPH, which is too low for realistic commuting. If we raised the voltage of the system, it would improve the power (for acceleration) and also the top-speed. Zero motorcycles use a nominal (average) voltage of 103V (roughly 117V when fully charged). Two of the Zero packs could be used in series for 206V nominal. The Zero packs are also known for being able to provide very high amps.

The 1,300 lbs of lead-acid batteries were in the front, right where it would make this prototype handle like a sports car! The super-cool haircuts reveal that these are real engineers, and not professional cigarette advertising models. 16M-520 is now the new password for my bitcoin account…

Adding roughly 40V to the original 168V system would definitely help (about 25%), but the amps from a modern Zero motorcycle pack would make a HUGE difference. You might be asking yourself why a big corporation like General Motors (who owns Opel, from Germany), would put any time and effort into a hybrid where their previous experience with electrics already told them the electric performance was going to be weak when using 1,300 lbs of lead-acid batteries.

[*Stir-Lec was 3100-lb, and the stock gasoline-engine 1968 Opel Kadett was 1780-lb]

The short answer is that, they were not testing how this would perform as an electric car…they were testing out the back-up generator that ran by burning any combustible fuel, and produced no smog.

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Stirling Engines

The “Stir” in Stir-Lec is for the Stirling engine. It took some digging, but I found out that GM was decided to leverage the existing experience that Philips had with developing Stirling engines (Philips was originally from the Netherlands). The particular Stirling engine that the Stir-Lec most resembles is a model that uses the “Beta” configuration, and specifically, the one in the Philips portable generator, the MP1002CA.

Philips had grown to a huge global corporation based on their design and production of radios in the 1930’s. There was a point when they felt they could sell more radios to customers in remote regions who had no grid electricity yet, by also selling them a generator where the engine was a small and modern version of the Stirling. It starts easily and runs very quietly.

The Philips MP1002CA Stirling Generator

In the pic above, the Beta style Stirling engine is everything on the right half of the machine. The horizonal cylinder on the top/left is the fuel tank which was typically filled with kerosene. A fan belt and pulleys connect the engine to the conventional generator on the bottom/left. The “cooler” section is merely a set of fan-cooled fins on the bottom half of the vertical cylinder of the engine.

The basic operation of a Stirling engine is very simple. First you use a cylinder that is hot at one end, and cold at the other. You insert a loosely-fitting metal balloon into the center, and this part is called the “displacer”.

When you move the displacer to the hot end, the gas inside the cylinder is forced into the cold end. The gas cools and the pressure in the cylinder goes down. Then, you move the displacer to the cold end of the cylinder, which forces the gas in the cylinder to move to the hot end. The gas then heats up, and the pressure in the cylinder rises.

If you then install a piston at one end of the cylinder, the rise and fall of the internal pressure will push the piston outwards, and a conventional crankshaft and flywheel will push the piston back inwards when the working gas has is cooled.

Desktop toy Stirling engines can demonstrate how they work while using air as the working gas. However, hydrogen or helium work much better as they absorb and shed heat very well, as well as them having no oxygen to cause oxidation of hot internal parts.

Another thing that can improve the power of a given Stirling engine is to pressurize the working gas. I would suggest that helium is readily available at these volumes, and if you pressurize the interior of the Stirling engine (perhaps to three atmospheres) it will cram additional helium into it, and that added mass of helium will provide additional heat-expansion from the same amount of fuel burned…

The pic above is the Philips Stirling generator in the Stir-Lec trunk. The chrome “coffee can” on top is the heater. The “regenerator” in the pic below is a spongy mass that gas can easily flow through and was made from aluminum oxide. It absorbs and sheds heat easily, and it increases the efficiency and power for a given size of engine.

The Stir-Lec Stirling engine, a Beta configuration designed and made by Philips.

Saying that a Stirling engine does not have any pollution in its exhaust is a bold statement. However, in 1968, this is the main reason GM was interested in the first place. The Environmental Protection Agency (EPA) was formed in 1970, but…even in 1968, GM knew which way government regulations were headed.

The average 4-stroke engine might be running at 2,000-RPM’s as it cruises down the highway. Since it has one “sparking event” at a frequency of once in every four strokes (two revolutions), there is a spark igniting some gasoline 1,000 times a minute. that is 1,000/60= 16 times a second!

As a car engine will speed-up and slow-down, the high frequency of the spark trying to properly ignite the gasoline is a demanding environment in which to produce clean exhaust. However, the Stirling generator runs at a constant RPM, and the hot end is heated by a constant flame that is adjusted to produce a clean burn. Also, since the hot end doesn’t care where the heat on the outside of it is coming from, a Stirling can use any fuel from propane to kerosene. This style is called “external combustion”, instead of the fuel being burned inside an enclosed cylinder, which is well-known as “internal combustion”.

A few years back, Jay Leno had one of his vintage steam-powered cars tested at a California smog station, and it passed all the modern requirements for a clean exhaust. Steam cars also use an external combustion flame, and it’s applied to the outside of a water-boiler.

With the previously-stated benefits, you may be wondering why Stirlings are not used in more applications. Their biggest flaw is that they are not very power-dense, meaning you would need a large Stirling engine to make a small amount of power. The Stirling in the Stir-Lec is a fairly sophisticated unit, and yet it is only rated for 8 Horse-Power at 2800-RPM’s.

Road-testing the Stir-Lec showed that it’s “stable system state” speed was 30-MPH. If the Stirling generator is running and the car is traveling at 30-MPH, the battery that they used was not draining or filling. This was a useful baseline performance level for this prototype. A short while later, GM was able to get ahold of some “next gen” Nickel-Metal Hydride batteries (NiMH) for a pure EV test, and that series of tests did actually end up with them producing 100 of the “EV1” in 1996, but they didn’t go back to testing a series-hybrid, which I think was unfortunate…

GM, Ford, Chrysler, and others all attempted to make a car that ran directly from a Stirling engine (click here), but even when the engine was enlarged as much as possible, acceleration was too slow. This is where a hybrid would shine. One of the best characteristics of electric drive is how much torque they have from the first RPM. Then, during the cruise-phase of driving, the energy drawn from the battery is quite low, so a back-up generator would not need to be large to simply keep it topped off.

My favorite configuration is a “plug in” hybrid. I love Tesla’s, and their extended range battery pack provides over 300 miles per charge. However, the larger battery pack is quite heavy and very expensive. For the vast majority of my driving, I only need about 100 miles of range. However, for any extended journey, a rarely-used generator could allow me to drive any distance I want.

In this scenario, I would charge up my car battery every evening in my garage, and I would drive it as an EV almost every day of the week. It would be very rare for me to ever need the generator to start and keep the hybrid running, but…it would be there if I ever needed it.

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Locomotives and Submarines

The submarines in WWII were configured as a series-hybrid. The diesel engines did not run the propellers directly at all, they only generated electricity to keep the large battery pack charged up. Also, after WWII was over, almost all the steam locomotives had been replaced by diesel trains, using a series-hybrid drivetrain.

The diesel-electric locomotives have a tiny battery, and the traction motors are driven directly from the generators on the diesel engines, through a controller.

The old submarines on the other hand, had a very large battery pack. They would typically cruise on the surface with the diesel running. If lookouts saw an enemy ship on the horizon, they would dive, turn off the diesels, and run silently on the battery during the attack.

The Stir-Lec was more like the Locomotives, where the engine would need to run all the time, because the battery was so small, but…modern lithium batteries would have solved that.

The car I envision is more like the WWII submarines, where it can run a long time as an electric only, but the engine can turn on to charge up the pack if needed. The fact that the Stirling runs clean and is a true multi-fuel engine is intriguing, but I am open to any clean-running engine that can spin the generator if a Stirling that fits is not powerful enough.

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Gotland-Class Submarines

In 1992, Sweden launched a new class of submarine. It is the usual vintage hybrid design that relies on electric batteries to run, and a snorkel that feeds air to a large and noisy diesel-generator to keep the battery topped off. However…

The Kockums V4-275R Mark-5 Stirling engine, rated for 75-kW of electricity generation

When they want to run silently without surfacing for an extended time, they have a large Stirling engine that also generates electricity to keep the batteries topped off (click here). These engines burn a mix of Liquid Oxygen (LOX) and diesel fuel to create heat, so they do not need a snorkel to feed air to an engine. Their large V4-275R Stirling engine would silently drive a 75 kW generator, and these submarines can run on the Stirling for roughly 14 days.

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More Stirling information

When steam engines were invented, they were immediately profitable for powering factories. However, business owners would often increase the pressure to get more power, instead of buying a larger steam engine. This would lead to horrific explosions, with death and injuries. The Scottish minister Robert Stirling wondered if he could design an engine that was safer.

The Stirling engines biggest benefit was also its undoing. It operates at a lower internal pressure compared to steam, and there was nothing that business owners could do to increase the stock power. They reliably run nearly silent, and they were very safe.

For a few years, Stirling engines enjoyed a great deal of success. However this competition spurred steam engine builders to institute training and licensing for steam engineers, which reduced the number of steam explosions. Because steam engines run at a MUCH higher pressure, you can get the same power from a smaller engine, so the steam engines of a given power range could be bought at a lower price than the Stirlings.

A Rider-Ericsson Stirling engine. The hot end on the right has a coal-stove built around the cylinder, and the cold side on the left has a shroud around it to hold water.

A Swedish engineer named Ericsson noticed that Stirlings could be safely run by anyone, without an expensive steam engineers license. He licensed the patent and designed an “Alpha” configuration heat engine in several sizes.

Hotels and Railroads bought thousands of them to pump water into elevated reservoirs. The railroads would gravity-fill steam locomotives with water from a tower, but there were many locations where there was not enough wind to pump the water up with a common windmill. The Rider-Ericsson Stirlings were perfect for this task.

The hotels used rooftop cisterns to provide gravity-feed water to their rooms. The water that was pumped would also cool the cold end of the Stirling, and as a result, that water was heated in the process.

I’ve been fascinated by Stirling engines as soon as I first found out about them. There have been Stirling generators that use the heat from a solar concentrating dish, with the cold-end being cooled by a simple air-fan and fins. Again, you would have to make a large Stirling to provide a small amount of power, but a Stirling could actually be made by any reasonably skilled mechanic using simple tools.

On a plug-in hybrid car with an 80-mile electric range, the engine is rarely run, if at all. A critic “could say” that burning bio-diesel or ethanol in a Stirling generator still creates “some” pollution, but the “PZEV” regulations (click here) show that clean-burning engines ARE possible. By having a hybrid, you get the performance of an electric vehicle, with a clean-running back-up generator to get you home on longer trips.

Don’t get me wrong, I LOVE electric cars. But building up the grid infrastructure to supply millions of EV’s will take time. We will get to a sustainable future eventually, but…I also have to get to work today. If I was buying an alternative-power vehicle today, it would be a Toyota RAV plug-in hybrid (click here).

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LINKS

Dec 1968 Popular Science magazine, pg 116 (click here)

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Written by Ron/spinningmagnets, March 2024

Grew up in Los Angeles California, US Navy submarine mechanic from 1977-81/SanDiego. Hydraulic mechanic in the 1980's/Los Angeles. Heavy equipment operator in the 1990's/traveled to various locations. Dump truck driver in the 2000's/SW Utah. Currently a water plant operator since 2010/NW Kansas

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