Chapter 4 Principles of Engine Operation 77 Copyright Goodheart-Willcox Co., Inc. long and narrow, which makes them relatively easy to remove and install. However, adequate engine compart- ment space may be a problem with this design. In addition to conventional vertical mounting, an inline engine can also be mounted on its side, such as in a bus where the engine is located beneath the passenger floorboard area. Horizontal, Figure 4-34, or flat inline engines may have all of the cylinders on one side or an equal number of cylinders on each side. When there are an equal number of cylinders on each side of the engine, it is known as a horizontally-opposed engine. TDC Four-stroke cycle Downstroke (power) Upstroke (exhaust) Downstroke (intake) Upstroke (compression) Expansion of gases BDC TDC Removal of gases Intake of air One revolution of crankshaft One revolution of crankshaft Two-stroke cycle 180° 360° 720° Two-stroke cycle Downstroke Upstroke Downstroke Upstroke BDC BDC Exhaust Expansion Scavenging Injection-ignition-combustion Expansion Exhaust Scavenging BDC Figure 4-32. Comparison of the two- and four-stroke cycles. Note the number of degrees it takes for each cycle. No 4. Cyl. No 3. Cyl. No 2. Cyl. No 1. Cyl. Figure 4-33. When the cylinders are placed in a single line, the engine is called an inline engine. Most modern inline diesel engines are six-cylinder. A B Figure 4-34. A—When the motion of the piston is horizontal, the engine is known as a horizontal engine. B—When cylin- ders are placed on each side of the crankshaft, the engine is horizontally-opposed. Compression of air Injection ignition combustion Compression Compression