OTTO FOUR-STROKE—CONTINUING THE TALE

THE OTTO engine’s four strokes (see yesterday, http://wp.me/p2ETap-2hj) involve two complete revolutions of its crankshaft, twice 360 degrees. From a timing point of view, the intake stroke is 0 to 180 degrees; compression is 180 to 360; ignition occurs around 360; the power stroke is 360 to 540; and exhaust is 540 to 720.

At its cylinder-head end, the Otto Silent Engine has two cams and a crank, all operated by a shaft driven by bevel gears off the engine’s crankshaft. It’s part of the engine’s charm that these are all exposed in operation.

Otto Silent Engine, detail of cylinder-head hardware. GMax model.

One of the cams operates the exhaust valve rocker arm; the other, a rocker arm controlling injection of the engine’s gaseous fuel. In fact, these early engines operated on Town Gas, the same stuff used in gas lighting. Today, a preferred fuel is hydrogen. The crank at the end of the shaft operates a slide valve serving both as intake valve and also as a means of ignition timing.

A slide valve, as viewed from the cylinder head. One of its pockets serves as intake valve, the other for flame-ignition timing. GMax model.

The slide valve has two pockets. One opens downward and receives the gaseous fuel just prior to the intake stroke. As the valve slides, this pocket meets a port in the cylinder head during the intake stroke.

The slide valve’s second pocket receives a tad of gaseous fuel and is then exposed to a flame, a pilot light, of sorts. As it continues its motion, this valve pocket of flame encounters the cylinder-head port, thus igniting the mixture in the combustion chamber.

It is a commentary of the late 1800s that any form of electric ignition was unreliable and inferior to this slide-valve flame ignition.

Slide-valve clearance with the cylinder head is evidently a crucial parameter. Too tight, and its sliding action is compromised; too loose, and it doesn’t seal. Four spring-loaded adjustors pressed against a plate fine-tune the desired clearance.

Head closeup. The four black knobs maintain optimal pressure on the slide valve. The Town Gas line feeds both the ignition pilot light and rocker-actuated fuel injector. GMax model.

Injection of the gaseous fuel into its slide-valve pocket is timed by its rocker arm actuated by the cam driveshaft. Otto engines operating today are fueled on hydrogen, typically piped in from a central source. (Such is the case at the Coolspring Power Museum; see http://wp.me/p2ETap-gD.) During the Gaslight Era of the late 1800s, these stationary internal-combustion engines ran on Town Gas, derived from coal and the same stuff piped through cities for illumination.

A talented modeler, Wayne Grenning, built an actual flame-ignition engine in 2013, a half-scale version of an 1883 French-built Deutz-license engine. Among its design complexities, he observes that flames “do not scale down properly.” Fifteen years of research followed. Wayne’s marvelous engine can be seen in operation at http://goo.gl/TPZzYu.

Here’s a video of my GMax model. Alas, it’s capable of only virtual action.

Nonetheless, there’s special pleasure (and challenge) in adapting GMax methods and Microsoft Flight Simulator capabilities to such a virtual model. Also, its videography is thanks to a software called Fraps.

More on GMax, Microsoft Flight Simulator and Fraps in the concluding item. ds

© Dennis Simanaitis, SimanaitisSays.com, 2014