Oilfield Engine News

Clerk and Reid Two-Cycle Engines


| November/December 2001



Clerk engine

Photo # 3: Longitudinal section view of Clerk engine.

Many folks who have oilfield engines either own or have owned a Reid. The Reid engine was apparently a very popular engine in it's day, as there are quite a few around as oilfield engines of that era go. The Joseph Reid Gas Engine Company was located in Oil City, Pa.

I have a Reid engine myself -it was the first oilfield engine I purchased, and my first exposure to an oilfield engine was a Reid at an engine show. The thing that most intrigued me about the Reid was the cylinder on the side. My first reaction was, 'this is a twin-cylinder engine,' but after I asked the owner he explained that it was a pump that forced the air and fuel mixture into the drive, or working, cylinder (see Photo #4 of Reid engine).

Now I really was interested in this thing, which to me sounded very unusual. Well, as things go and time progressed, 1 got my very own Reid, and the purpose of this month's column is to share with you some of the things 1 have learned about the unusual little 'pump' cylinder on a Reid.

In the course of my limited research and digging around, and assuming I understand properly, a Reid engine operates on what is known as the 'Clerk' cycle. The Clerk cycle is named for Sir Dugald Clerk, born in 1854 in Glasgow, Scotland, and knighted in 1917, apparently for his work in developing the two-cycle engine. A Reid is a two-cycle engine of a different sort.

Developed by Sir Clerk in an early form in 1878 (see Photo #1), the Clerk engine featured two cylinders, one being a pump cylinder, the other a motor cylinder. The pump crank followed the motor crank, and it pumped a mixture of gas and air into an intermediate reservoir through a check-valve. From this reservoir the mixture was supplied to the motor cylinder by means of a slide-valve. The mixture was compressed in the reservoir to a pressure of 70 psi. This arrangement allowed an impulse (power stroke) every revolution. In the sectional plan of the Clerk gas engine (see Photo #2), 'A' is shown to be the drive cylinder, 'B' to be the charge or pump cylinder, 'C' to be the drive or working piston, and 'D' to be the pump or charge piston - nearly exactly the arrangement of a Reid engine.

I do not know how Joseph Reid came about using this design, neither do I know of any other makes of engines that operate as this. Maybe someone else can shed further light on this. The longitudinal section of the engine shows the 'reservoir' (P), which was pumped up to 70 psi for fuel and air to supply the drive cylinder on each cycle. This is something that was excluded on Clerk's future designs. I'd assume there was a safety concern here also, as back ignition into the reservoir through the check-valve could result in a nasty explosion. Anyone who has had trouble with the check-valve leaking on their Reid can understand this as back ignition can occur in the pump cylinder. This problem more than likely was never totally resolved, as most Reids have a pop-off valve in the end of the charge cylinder to prevent damage to the engine in cases of back ignition into the pump cylinder.