content_image/591980.jpg
adjusted idle mixture setting, it is possible to run the engine at idle rpm for long periods. Such a setting results in a minimum of plug fouling and exhaust smoking, and it pays dividends from the savings on the aircraft brakes after landing and while taxiing. 

If the wind is not too strong, the idle mixture setting can be checked easily during the ground check as follows: 

1. Close throttle. 

2. Move the mixture control to the idle cutoff position and observe the change in rpm. Return the mixture control back to the rich position before engine cutoff. 

As the mixture control lever is moved into idle cutoff, and before normal dropoff, one of two things may occur momentarily: 

1. The engine speed may increase. An increase in rpm, but less than that recommended by the manufacturer (usually 20 rpm), indicates proper mixture strength. A greater increase indicates that the mixture is too rich. 

2. The engine speed may not increase or may drop immediately. This indicates that the idle mixture is too lean. The idle mixture should be set to give a mixture slightly richer than best power, resulting in a 10- to 20-rpm rise after idle cutoff. 

## Engine Stopping 

With each type of engine installation, specific procedures are used in stopping the engine. The general procedure, outlined in the following paragraphs, reduces the time required for stopping, minimizes backfiring tendencies, and prevents overheating of tightly baffled air-cooled engine during operation on the ground. 

In stopping any aircraft engine, the controls are set as follows, irrespective of the type or fuel system installation. 

 1. Cowl flaps and any other shutters or doors are always placed in the full open position to avoid overheating the engine and are left in that position after the engine is stopped to prevent engine residual heat from deteriorating the ignition system. 

2. Carburetor air-heater control is left in the cold position to prevent damage that may occur from backfire. 

3. Constant speed propeller is usually stopped with the control set in the high pitch (decrease rpm) position. 

No mention is made of the throttle, mixture control, fuel selector valve, and ignition switches in the preceding set of directions because the operation of these controls varies with the type of carburetor used with the engine. An engine 

equipped with a carburetor incorporating an idle cutoff mixture control is stopped as follows: 

1. Idle the engine by setting the throttle for 800 to 1,000 rpm. 

2. Move the mixture control to the idle cutoff position. In a float-type carburetor, it equalizes the pressure in the float chamber and at the discharge nozzle. 

3. After the propeller has stopped rotating, place the ignition switch in the off position. 

In addition to the operations outlined previously, check the functioning of various items of aircraft equipment, such as generator systems, hydraulic systems, etc. 

## Basic Engine Operating Principles 

## Combustion Process 

Normal combustion occurs when the fuel/air mixture ignites in the cylinder and burns progressively at a fairly uniform rate across the combustion chamber. When ignition is properly timed, maximum pressure is built up just after the piston has passed top dead center at the end of the compression stroke. 

The flame fronts start at each spark plug and burn in more or less wavelike forms. [Figure 10-42]  The velocity of the flame travel is influenced by the type of fuel, the ratio of the fuel/air mixture, and the pressure and temperature of the fuel mixture. With normal combustion, the flame travel is about 100 feet/second. The temperature and pressure within the cylinder rises at a normal rate as the fuel/air mixture burns. 

<img src='content_image/591980.jpg'>

