Not many engines still run when they are 125 years old, but this one does. Built in 1897, this Mietz and Weiss 6-horsepower oil engine is on display in the Founder’s Building at the Coolspring Power Museum. The copper tank contains the fuel oil. M&W recommended kerosene, which was probably the best, cleanest and most standardized fuel available in the day. The M&W could run on other fuel oils but operators soon learned they coked up rather quickly, as did other makes, but were more difficult to clean out than the others.

1897 Mietz and Weiss Oil Engine

In the late 1800s and early 1900s, Rudolph Diesel had a worldwide stranglehold on patents related to his four-stroke compression ignition cycle. Engineers found they could get partway there without being sued for a patent violation and that’s how the oil engine came to be.

We talked about Herbert Stuart-Akroyd and his invention of the four-stroke vaporizing oil engine in England (see it at and how that engine offered a lower cost engine that could operate on a variety of heavy fuel oils. An enterprising German emigrant named Carl W. Weiss did something similar in the USA, with a few clever tweaks.

Mietz and Weiss

Carl Wilhelm Weiss (1858-1940) was born in Prussia and attended engineering school in Holland. He travelled to the U.S. in 1876 to see the 1876 Centennial Exposition in Philadelphia, Pennsylvania, and decide to stay. Finding employment as an engineer, he patented a number of items outside the engine realm, including a pneumatic typewriter and engineered things elated to cash registers and adding machines, but internal combustion engines had captivated his interest. He had the knack and by1893 had developed a vaporizing oil engine. Lacking funds for a full startup, he began looking for investors.

Not many engines still run when they are 125 years old, but this one does. Built in 1897, this Mietz and Weiss 6-horsepower oil engine is on display in the Founder’s Building at the Coolspring Power Museum. The copper tank contains the fuel oil. M&W recommended kerosene, which was probably the best, cleanest and most standardized fuel available in the day. The M&W could run on other fuel oils but operators soon learned they coked up rather quickly, as did other makes, but were more difficult to clean out than the others.

Weiss soon teamed up with August Mietz (1834-1917) of New York City. Mietz was also a Prussian with an engineering background. He had emigrated to the U.S. in 1859 and by the time he and Weiss started working together, he had a large and well-established foundry/machine shop on Mott Street in Lower Manhattan, New York. It was the perfect place to put Weiss’ designs into production and it was done quickly. As far as we can tell, the Mietz & Weiss (M&W) engines were the first American designed oil engine to go on sale here. Weiss continued perfecting the designs and ran the technical side of M&W for many years. The partnership of Mietz & Weiss produced oil engines in a variety of styles and outputs, as well as gasoline engines and, reportedly, alcohol fueled engines.

Oil and Water- Hit and Miss

The Mietz & Weiss oil engines were two-stroke, hit and miss, crankcase scavenged, vaporizing oil engines. They were the first American hot bulb engine and utilized the vaporizing principle first used by Akroyd-Stuart. Well, that doesn’t tell you a lot, does it, but looking at the nearby illustration and reading the caption will tell you more.

Oil engines are not “diesels” but they are on the diesel family tree. In concept, they are most like the prechamber style, indirect injected (IDI) diesels of the recent past but with very low compression ratios (3:1 to 5:1). In fact, the oil engine developed into the IDI diesel. Oil engines use heat of compression for ignition and they are (generally) fuel injected, but they need a heat source in the combustion chamber to vaporize the oil into a more volatile mixture. This is where the hot bulb comes in. The hot bulb is externally heated with a torch initially but after the engine heats up, it stays hot enough to sustain vaporization. The bulb is not cooled at all by the engine cooling system, so it can retain heat for vaporization. As the oil engine evolved into the diesel, compression ratios got higher and the vaporizing chamber got smaller.

The lightbulb-shaped, cast iron hot bulb sticks out of the cylinder head and lives inside a cast iron housing. It has cover (2), sometimes called the chimney. The top cover is removed to heat the bulb with a torch for starting and warmup and reinstalled to hold heat as the engine runs, after the external heat is removed. The injector (1) is a simple affair. The earthy Mike Murphy describes injection pressure as, “About as much pressure as you can generate making a loogie.” A fuel oil loogie: Got it Mike! The muffler is a period type.

The M&W oil engines were unique in using water injection to slow combustion and reduce knock. The first M&W oil engines used a manually adjusted water injection system. Later ones had a “toilet tank” style system with a float valve to automatically adjust water feed. The water jacket around the cylinder was used to heat water into low pressure steam, about as much as comes of a teapot, that was pushed into the intake tract at slightly more than atmospheric camber. The engine needed a supply of water, either from a pressure-regulated city water system or a gravity tank. Running at light loads, the engines do not need the water injection and typically modern hobbyists do not use it. Under load, it’s pretty important.

Here we see the cooling and water injection components. The water tank (1) connects with the water jacket around the cylinder and is fed by a water line. Inflow is controlled by a float valve, similar to what you may find in your toilet tank. The steam expands into the copper “turnip tank” (we don’t know it’s actual nomenclature) at the top of the water jacket and if you follow the copper line, you can see where it feeds into the intake flow. Under a light load, the steam is not needed, so operators find the engine runs better if left to run hot.

The M&W oil engine was unusual in another way; it was a hit and miss. Many large flywheel gasoline engines of the day were hit and miss, but the M&W is the only oil engine we can find with that feature. Basically, hit and miss is a way of governing engine speed. If the engine has a carburetor with a throttle plate, a governor can control speed with it. Gasoline hit and miss engines speed are speed controlled by cutting off the spark. The heavy flywheel keeps the engine rolling until speed drops  to the governed point and then cuts in the spark. A hit and miss oil engine works the same way except that the fuel injection is cut off. With either style, with a light load and depending on where the governor is set, a hit and miss might fire once or twice in every five revolutions. Under a heavy load, it will usually fire every revolution.

This Old Engine

The engine featured here has been dated at 1897. It’s serial number 579 and is one of the earliest surviving and running M&W oil engines. It can been seen in the Founder’s Building at the Coolspring Power Museum, in Coolspring, Pennsylvania. It’s a 6 horsepower model with a 7 x 8 inch bore and stroke. It was used on a Farm in Claysville, Pennsylvania, and has been at the museum since 1970. During events there, it is often found hitting and missing for days on end.

The M&W Legacy

Mietz & Weiss was a going concern in the 1890s and early 1900s. Their oil engines were at the forefront of the technology for that time and had a great reputation for reliability. Besides single-cylinder horizontal oil engines from 1-1/2 to 35 horsepower, they built gasoline engines from 1/2 to 18 horsepower. Their horizontal twin cylinders produced up to 70 horsepower. M&W were also known for vertical engines, particularly their vertical marine oil engines from one to four-cylinders and 2 to 80 horsepower.

Here’s how the M&W hot bulb system works. Let’s say the engine has just fired and the piston is on its way back towards BDC. As the piston goes down the bore, the Exhaust Port (marked G- in red) is uncovered and all combustion pressure is released. Very soon the Air Port (marked in blue) is uncovered as well. Note the Suction Port at the bottom of the cylinder towards the crankcase. When the piston is towards the top of the stroke, it’s uncovered, letting atmospheric pressure fill the crankcase with air from the air chamber in the engine base. As the piston moves down, it covers the Suction Port and also decreases the volume of the crankcase (which is sealed), increasing it’s pressure and when the Air Port is uncovered, that pushes air into the cylinder… but not just air! The turnip shaped vessel marked E in green is plumbed to the cooling jacket (fed by a tank on the opposite side of the engine) and as the engine heats up, the water is turned to light steam and at the right time, it’s injected into the incoming airflow via the tube marked F. When the piston starts back up to TDC, compression takes place. The compression pressure of the M&W was higher than most engines of the era, 60 psi or about 4:1. When the piston nears TDC, the fuel is injected at B. It hits that little lip/cup below, which is a part of the hot bulb vaporizing chamber (marked C). The piston moving forward pushes some of that fuel into the hot bulb, where its instantly vaporized by heat and turned into a volatile mixture ready to ignite. That 4:1 compression ratio completes ignition and as the flame front travels out of the bulb, it ignites the rest of the fuel in the combustion chamber and the process begins again. So what was the water all about? While this ignition process is slow by today’s standards, combustion still happens fast and hits hard. This especially true of kerosene, the recommended fuel for the M&W oil engines. The water vapor slows combustion a little and reduces knock. Engine durability at the cost of a little power. Also note item D at the bottom of the cylinder, the kerosene torch used to heat the hot bulb. The cover over the hot bulb is actually a chimney, so it must be open to heat the bulb with the torch but closed to hold heat once the torch is shut off and the engine is warmed up.

It’s known that some time before 1908, Mietz semi-retired and left the day-to-day operation of the company to his son-in-law, Emil Rueff. Sources conflict but Meitz died in either 1915 or 1917. Carl Weiss, who is sometimes called the “Dean of Oil Engineers,” left the company in 1917 and started his own engine manufacturing outfit called Weiss Engine Company in 1918. It didn’t go very far and some sources state he ran afoul of patents owned by August Mietz Company and had to settle a lawsuit. Wiess finished his career running the Carl W. Weiss Engineering Company, which was in operation as late as 1939, though little is known about what was done there.

With both Weiss and Mietz gone, the company slowed down and some time between 1920 and 1922, it shut down and Charter Gas engine Company acquired the rights to some M&W designs and began manufacturing them in Illinois. Charter went out of around 1930 and still had M&W oil engines in the catalog right to the end. Interestingly, the ornate Mietz Factory, still called the Mietz building, exists on Mott Street in Manhattan, long since repurposed into offices. It gained another claim to fame by serving as a backdrop in the mafia movie, The Godfather.



Coolspring Power Museum


You May Also Like


The ’70s TV talk show host Tom Snyder was known for asking, “What goes through their minds?” when confronted with inexplicable human behavior. That might apply when pondering why GMC…


Following the installation of a new Class V hitch, trailer brake controller, and free-flowing exhaust system in Part Three, this time we’re replacing another vital component in our ’97 F-350’s…

The Legacy of Caterpillar D364: Powering the Great Lakes and Beyond

In the ‘30s, Caterpillar entered the diesel electric generator market. Cat assembled the units in house, sourcing the generator heads from outside the company. Caterpillar was…

The Perky Massey

The Massey-Ferguson 65 Diesel debuted for 1958. The DNA for that tractor started with the 1939 intro of the Ford-Ferguson 9N, a brilliant tractor designed largely by Harry Ferguson but…