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Spot the oddity

station Road Steam item. Open Crank IC engine.

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Bryan Cedar 107/11/2021 15:10:09
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Saw this IC engine in Station Road Steam site and though the valve gear looked very odd. Can you spot the oddity? Nothing mentioned in the description.

https://www.stationroadsteam.com/open-crank-ic-engine-stock-code-9530/

JasonB07/11/2021 15:24:07
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I can't spot anything, looks the common arrangement for hit and miss engines. Cam behind the timing gear moves the push rod, that moves the rocker and that moves the exhaust valve. Inlet is atmospheric and just opens when the vacuum cause by the descending piston sucks it open.

Bryan Cedar 107/11/2021 15:52:25
98 forum posts
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Posted by JasonB on 07/11/2021 15:24:07:

I can't spot anything, looks the common arrangement for hit and miss engines. Cam behind the timing gear moves the push rod, that moves the rocker and that moves the exhaust valve. Inlet is atmospheric and just opens when the vacuum cause by the descending piston sucks it open.

Hi Jason

You have answered my query. ie. there is no means of operating the inlet valve. did not realise it is operated by vacuum. Many thanks. Matter closed.

JasonB07/11/2021 16:01:51
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Before someone picks me up on it I should say it is not just hit and miss engines that can have this sort of inlet valve, many of the early engines did whatever means they were governed by or those that did not have a governor. does make it a bit easier to set them up, the spring is a lot weaker than that on the exhaust as it just needs enough strength to return the valve to the closed position

This is one of my engines being turned over by hand and you can see and hear the valve opening, the exhaust is on the side of the cylinder so you can't cee the pushrod operating that.

Howard Lewis07/11/2021 18:43:13
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As Jason says, a lot of early engines had suction operated Inlet valves. The Inlet valve spring was a light one with a low rate and minimal preload, so that the valve opened with minimal depression in the inlet tract.

The exhaust valve was operated by the push rod, with the governor weights in the flywheel preventing the exhaust valve being opened when the engine speed rose too high.

Hence the "Hit and Miss" description applied to them.

The governing system is vastly improved,(operating on the throttle butterfly ) but the modern equivalent is the reed valve ; found on a lot of small engines on lawn mowers, rotavators, and the like.

Howard

JasonB07/11/2021 18:54:57
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Posted by Howard Lewis on 07/11/2021 18:43:13:

 

The exhaust valve was operated by the push rod, with the governor weights in the flywheel preventing the exhaust valve being opened when the engine speed rose too high.

Most tend to hold the exhaust valve open rather than closed, that means the air is sucked in through the exhaust so no fuel/air mix enters via the inlet so the engine misses that ignition cycle.

You can see me manually doing what the governor does by holding the exhaust valve open on this test run as I had the governor disabled for initial tests. Inlet valve can also be seen working but not when exhaust held open.

 
And this one shows as similar governor arrangement to the station road steam model working by holding the pushrod in the partially open position
 

Edited By JasonB on 07/11/2021 18:55:38

martin perman07/11/2021 19:24:00
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I own a 1924 Ruston Hornsby 2 1/2 HP open crank with the vacuum operated inlet valve and I use the valve to allow me to crank the engine up without trying to overcome the compression to start it.

Martin P

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