What depression exists in the cylinder when the scavenging ports open in the Simplic engine?

The gas expansion has gone so far that a depression of 0.035 to 0.05 atmospheres below ambient is present in the cylinder, causing atmospheric air to flow in through the bell-shaped inlet chest.

How is exhaust scavenging vacuum produced in the Simplic engine?

By connecting two cylinders, with explosions displaced by 180° of crank angle, into a single branch of the exhaust pipe, so exhaust gas velocity develops a depression to scavenge the second cylinder.

What are the port and valve timing figures given for the Simplic cycle?

The piston-controlled ports close about 55° past bottom dead centre, the exhaust valve closes 115° past bottom dead centre, and the second cylinder's exhaust valve opens 50° after the first cylinder's valves close.

What is the mean effective pressure of the Simplic engine?

Only about 40 or 50 lb/in², which is lower than a conventional arrangement with the same maximum pressure — the main disadvantage of the more-complete-expansion cycle.

What output could an aluminium V4 Simplic engine produce?

The document states an aluminium V4 weighing 220 lb might produce 40 bhp fairly reliably, thanks to the engine's lightness and compactness.

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Chatterton Simplic Two-Stroke Engine - Some Unusual Engines, Page 44 (2 of 3)

Engineering book page on unusual two-stroke engines, scavenging and compression ratio

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Page 44 from an engineering text 'Some Unusual Engines' discussing two-stroke port scavenging, cylinder vacuum 0.035-0.05 atm, exhaust timing and expansion vs compression ratio.

AI caption January 2014 · Photo 424 of 465

This is page 44 from a book chapter titled "Some Unusual Engines," describing the operating cycle of the Chatterton "Simplic" two-stroke engine (per the uploader's filename, this is page 2 of 3). The text explains how, when inlet/scavenging ports open in the cylinder wall, a depression of 0.035–0.05 atmospheres below ambient draws fresh air into the cylinder through a bell-shaped inlet chest, promoting swirl at the piston crown. Exhaust scavenging is aided by pairing two cylinders (explosions 180° apart in crank angle) into one exhaust branch. Key timing figures are given: piston-controlled ports close about 55° past bottom dead centre; the exhaust valve closes 115° past BDC; the second cylinder's exhaust valve opens 50° after the first cylinder's valves close. The page explains that delayed exhaust-valve closure creates a more-complete-expansion (hyper-expansion) cycle in a two-stroke, analogous to early inlet closing in a four-stroke, yielding higher efficiency but lower mean effective pressure — only about 40–50 lb/in² in the Simplic. It concludes that an aluminium V4 version weighing 220 lb might reliably produce 40 bhp. This is a general engineering reference, not a Ford Fairlane service document.

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Frequently asked questions

What depression exists in the cylinder when the scavenging ports open in the Simplic engine?: The gas expansion has gone so far that a depression of 0.035 to 0.05 atmospheres below ambient is present in the cylinder, causing atmospheric air to flow in through the bell-shaped inlet chest.
How is exhaust scavenging vacuum produced in the Simplic engine?: By connecting two cylinders, with explosions displaced by 180° of crank angle, into a single branch of the exhaust pipe, so exhaust gas velocity develops a depression to scavenge the second cylinder.
What are the port and valve timing figures given for the Simplic cycle?: The piston-controlled ports close about 55° past bottom dead centre, the exhaust valve closes 115° past bottom dead centre, and the second cylinder's exhaust valve opens 50° after the first cylinder's valves close.
What is the mean effective pressure of the Simplic engine?: Only about 40 or 50 lb/in², which is lower than a conventional arrangement with the same maximum pressure — the main disadvantage of the more-complete-expansion cycle.
What output could an aluminium V4 Simplic engine produce?: The document states an aluminium V4 weighing 220 lb might produce 40 bhp fairly reliably, thanks to the engine's lightness and compactness.