Lesson 3. Two-stroke and Four-stroke Diesel Engines

TWO-STROKE AND FOUR-STROKE DIESEL ENGINES

Any internal combustion engine, regardless of principle it operates on, has a four-stroke or a two-stroke cycle. All modern diesel engines are single-acting, i.e. there is only one combustion space, above the piston. The double-acting principle, i.e. with a combustion space both above and below the piston, was also popular for many marine engines. But engines employing this system are now obsolete because of their relatively complex design and difficulties in maintenance.

The four-stroke cycle consists of the suction stroke, compression stroke, combustion and expansion stroke, and exhaust stroke. The piston starts downward, suction stroke. The air inlet valve is open and air is drawn into the cylinder through the air inlet pipe. The exhaust valve, fuel valve are all closed. As the piston reaches the end of the suction stroke the air inlet valve closes and as the piston rises on the second, compression stroke, the air in the: cylinder is compressed to about 480 pounds and its temperature rises to about 1.000 F. The fuel injection valve now opens and the fuel oil is sprayed into the cylinder under a pressure of 3.550 p.s.i. The high temperature of the compressed air in the cylinder ignites the fuel, and it continues to burn as long as injection is maintained. The pressure in the cylinder, increases and the piston is driven downwards. This is the third, power stroke. The injection valve closes shortly after the piston has started down on this stroke. At the end of this stroke the exhaust valve opens and the burned gases in the cylinder, now reduced to 40 pounds pressure, start to flow out through the exhaust pipe. Returning on the fourth, exhaust stroke, the piston pushes the remaining gas out of the cylinder, through mechanically operated exhaust valves. The complete cycle of operation thus requires four stokes of the piston, and in each cylinder there is one power stroke during two revolutions of the crankshaft.

In the two-stroke engine the complete cycle of operation takes place duringone revolution of the crankshaft: 

1) the up stroke, the scavenge and compression stroke

2) the down stroke of the piston being, the power and exhaust stroke.

Instead of an exhaust valve there is ring of exhaust ports around the bottom of the cylinder, communicating with the exhaust pipe. In place of air inlet valves there are scavenging ports. When the piston on its downward stroke uncovers the exhaust ports, and the cylinder pressure drops to atmospheric, they open and the air under pressure flows into the cylinder and pushes the exhaust gases out through these ports. As the piston on its up-stroke covers the scavenging ports, the exhaust ports close, leaving the cylinder full of fresh air. Just before the end of the compression stroke, fuel is injected. As it burns, and the gases expand within the combustion chamber, the piston is driven down on its power stroke. Although with the two-stroke engine there are twice as many power strokes per revolution as in the four-stroke engine, the power developed is not doubled by using the two-stroke cycle. Power is absorbed in pumping losses. Moreover, the power stroke is partly lost for exhaust and scavenging. The diesel engines may also be op­posed  piston  type,  crosshead  type  and  trunk  piston  type.  Large diesel engines, which have cylinder near 900 mm in diameter turn at the rela­tively slow speed at about 145 rpm and less.  They are  known  as  slow speed diesel engines.  At this low speed the engine requires no reduction gearbox between  it  and  the  propeller.  Medium  speed  diesel  engines operate  between  150  and  450  rpm,  therefore  they  are  connected  to  the propeller by gearing.

I. Read the text and translate the following sentences into English.

1. Всі сучасні дизельні двигуни - це двигуни простої дії.

2. У двигунів простого дії всього одна камера згоряння.

3. Дизельні двигуни також можуть бути двигунами з протилежно рухомими поршнями, крейцкопфними двигунами, тронковими двигунами.

4. Великі дизельні двигуни обертаються на низькій швидкості.

5. Низькооборотні двигуни зазвичай мають циліндр діаметром 900 мм.

6. У низькооборотних двигунів немає понижуючого редуктора.

7. Середньооборотні двигуни працюють на швидкості 150-450 оборотів в хвилину (rpm - revolutions per minute).

8. Середньооборотні двигуни з'єднані з гвинтом зубчастою передачею.

9. Чотиритактні суднові дизелі поділяються на головні і допоміжні.

10. Головні дизелі є джерелами енергії для приводу рушіїв.

11. Допоміжні дизелі виробляють енергію для роботи електричних генераторів, компресорів та інших механізмів.