displaced by the multiple cylinder form. Racing automobiles have been made with eight, twelve, and sixteen cylinders, but eight cylinders have been the greatest number ever used on a stock touring car.

Q. What is the advantage of a one-cylinder engine?

A. The main advantage of a single cylinder motor is that of simplicity. Practically no one-cylinder automobiles are on the market at the present time, though many one-cylinder cars of early vintage are still giving satisfactory service.

Fig. 18.-Exhaust Side of Four-Cylinder Water-Cooled Motor Used on Peerless Automobiles.

Q. Why are multiple cylinder engines superior to simple types?

A. Multiple cylinder engines are smoother running because they deliver power more uniformly than do the simple one and two cylider engines.

Q. How many explosions per crankshaft revolution are obtained with various forms of four-cycle engine?

Fig. 21.-End Sectional View of Motor With T Head Cylinder Showing Important Parts.

A. The crankshaft of a single cylinder four-cycle engine will receive one impulse every two revolutions. That of a two cylinder form will receive an impulse every revolution. A three cylinder crankshaft will receive three impulses in two revolutions or one every two thirds crankshaft revolution. Four explosions are obtained for every two revolutions of a four cylinder crankshaft or two each

revolution. Three impulses are given to the crankshaft of a six cylinder engine per revolution and that of an eight cylinder motor receives four impulses per crankshaft revolution. It will be apparent that the greater the number of cylinders the smoother the engine operation will be because the crankshaft receives more power strokes in a unit of time. A two-cycle motor crankshaft will receive twice as many impulses per revolution as that of a four-cycle engine having the same number of cylinders.

Q. Are five or seven cylinder engines practical?

A. Five cylinder engines of the conventional form having five throw crankshafts have been made and have operated satisfactorily though they do not run enough smoother than a four cylinder to warrant their replacing this type of standard engine and at the same time their torque is not equal to that of a six cylinder engine, so they have not been applied to automobile service. Any number of cylinders that will go into 360 evenly can be utilized in conjunction with a crankshaft having the same number of throws as there are cylinders, and these cranks may be arranged so that the explosions will follow each other evenly. A seven cylinder engine cannot be made of the usual form, but has been made with cylinders revolving around a fixed crank for aeroplane use. A five cylinder revolving engine was formerly utilized as a power plant on an automobile sold in limited numbers.

Q. What is the limit of engine weight per horse power?

A. The ordinary automobile power plant will weigh about ten pounds per horse power, but very light gas engines for use as aeroplane power plants have been made in the water cooled form that weighed less than five pounds per horse power complete and in the air cooled revolving cylinder forms that weighed but three pounds per horse power. Automobile power plants have been improved greatly in recent years, as 25 or 30 pounds per horse power was not considered excessive weight when the first gas engines were applied to automobile service.