M1: Internal Combustion Engines Flashcards

Question

Brake Power

Answer 1

This is the power available for doing work at the output shaft of the engine. It is indicated power minus losses due to friction, pump drives, and blowers.

Answer 2

Represents the ratio of work done by an engine at the flywheel to the work done in the cylinder. Losses in mechanical efficiency are due to friction between bearings and running surfaces, and the power required to drive blowers, pumps, and camshafts.

Answer 3

Represents the work done at the flywheel of an engine in heat units to the energy available to the cylinders from the fuel. Energy losses are due to exhaust, cooling water, and radiation.

Answer 4

Also known as brake specific fuel consumption (BSFC) it is a measure of the fuel efficiency of any prime mover that burns fuel and produces rotational or shaft power. It is the rate of fuel consumption divided by the power produced. It is typically used for comparing the efficiency of internal combustion engines with shaft output and allows the fuel efficiency of different engines to be directly compared.

Answer 5

- Most compact, consumes less oil, can safely operate at high speeds. - More complicated, develops less horsepower per pound. - Cycle divided into 4 events: air inlet, compression, power/ working stroke, and exhaust. - Fitted with exhaust and intake valves in the cylinder head to allow for gas exchange process, actioned by a cam shaft that is driven by the crankshaft. The timing is determined by the profile of the cam and the position of the cam relative to the crank angle.

Answer 6

At 15 deg before TDC, inlet valve begins to open to allow time for it to be fully opened and have a more effective intake stroke. The valve closes at 5 deg past BDC to increase efficiency of intake stroke due to the air entering having inertia and it will draw more air into the cylinder. The pressure differential between the increasing volume cylinder and atmosphere will draw in air.

Answer 7

The compression stroke commences as soon as the intake valve is fully closed. The air trapped in the cylinder is compressed by the piston which is moving upwards and reducing the volume of the cylinder. The purpose of this stroke is to heat the air charge by rapid compression, allowing ignition of an injected quantity of atomized fuel. The temperature and pressure of compression vary from engine to engine, but some average figures would be 45 bar and 650C (medium speed) and 35 bar and 540C (slow speed).

Answer 8

At 10 deg before TDC, fuel injected into the combustion chamber by the fuel pump and injector. The fuel is at sufficient pressure to penetrate the dense air charge and is in a atomized form to allow it to mix thoroughly with the air. The fuel is injected before TDC because there is an ignition delay period between injection and ignition. The entire fuel charge is injected over a period of crank angle, the length of the period depending on the amount of fuel which must be injected (engine load) however it usually ends around 35 deg after TDC. This period of injection tends to maintain a near constant pressure in the cylinder during the first part of the expansion stroke (this produces the characteristic high torque of the diesel engine). As the piston moves downwards in the cylinder during the expansion stroke the increasing volume of the cylinder causes the pressure to drop. Firing pressures vary greatly depending on the engine type but vary from 40 bar to 50 bar. Firing temperatures can reach 1300 C.

Answer 9

The exhaust stroke begins when the exhaust valve is opened at 35 deg before BDC. Pressure in the cylinder will have fallen to about 3 bar at this point and this part of the exhaust stroke is referred to as the blow down period. Pressure in the cylinder is allows to fall to exhaust manifold pressure between the time the exhaust valve opens and BDC, thus reducing pumping losses during the exhaust stroke. During the exhaust stroke, the piston decreases the volume of the cylinder creating a pressure higher than that in the exhaust manifold. This will cause exhaust gases to be expelled via the exhaust valve. The exhaust valve is closed at 10 deg after TDC to allow the full exhaust stroke to be utilized.

Answer 10

There is a period of crank angle when both the exhaust and intake valves are open. This is known as valve overlap and all four stroke engines have it to some degree. Supercharged engines generally have a longer valve overlap period than normally aspirated engines. Valve overlap increases the efficiency of the gas exchange process (increases volumetric efficiency). The exhaust gases are leaving the cylinder by flowing through the exhaust valve, when the intake valve is opened slightly before TDC. This movement of exhaust gases induces intake air to flow into the cylinder as soon as the intake valve opens, mixing with the exhaust gases. This induction of intake air scavenges the clearance volume, flushing out exhaust gas which would otherwise be trapped. It also starts the flow of intake air into the cylinder before the piston commences its downward intake stroke.

Answer 11

- The thermal and mechanical efficiencies are slightly better so fuel consumption per unit power output is slightly less - Only half the quantity of heat generated has to be dealt with in a given time, so efficient lubrication of the pistons and cooling of the cylinders is more easily accomplished

Answer 12

-Up to 1.8 times more powerful than a 4-stroke. - Large stroke provides maximum efficiency and it produces a large power output as well as easily burning heavy fuel. Slow speed eliminates the need for a reduction gearbox - Divided into two distinct events: power/working stroke and compression. - The gas exchange process must be carried out during a very short crank angle at the bottom of the piston stroke. - Require air pumps or blowers to generate the energy to carry this out. The pumps may be exhaust gas drive, and engine or separately driven - Entry and exit of gases can be provided in two ways: by using a ported liner (openings in the cylinder) or by a combination of ports in the cylinder and an exhaust valve in the head

Answer 13

At 10 deg before TDC, fuel is injected into the combustion chamber by the fuel pump and injector. The fuel is injected through a period of crank angle tending to maintain a constant pressure during the initial period of the stroke (generally ending at 15 deg TDC). The burning fuel charge causes the pressure and temperature to rise and the expanding gases act on the piston during the downward working stroke. Average pressures and temperatures would be 40 bar and 1300C As the piston increases the cylinder volume, the pressure falls. At 45 deg before BDC, either the exhaust valve in the cylinder head begins to open or the top ring on the piston uncovers the exhaust ports in the liner. When this occurs, pressure in the cylinder is approximately 3 bar and it falls rapidly to exhaust manifold pressure (blow down period). At 40 deg before BDC, the top ring on the piston will uncover the scavenge port, the pressure in the scavenge manifold will be greater than the pressure in the cylinder and scavenge air will enter the cylinder flushing the exhaust gases out of the exhaust passages. The scavenge period occurs between the time the top piston ring uncovers the scavenge port on its downward travel and when it covers the port on its upwards travel. During this time, the cylinder is flushed of exhaust gases and recharged with oxygen rich air. The measure of the quality of this gas exchange process is called the scavenge efficiency of the engine. If the engine is fitted with an exhaust valve, then this valve is closed first (35 deg after BDC), allowing the scavenge ports to stay open longer and producing a supercharge in the cylinder. If the engine has fully ported liners, then the scavenge ports will of course close first, making it more difficult to supercharge the cylinder since the exhaust ports are still open.

Answer 14

After the scavenge port has closed in the engine fitted with an exhaust valve, or the exhaust port has closed in the engine fitted with a fully ported liner, the compression stroke begins. Air trapped in the cylinder is increased in pressure and temperature due to the decreasing cylinder volume caused by the piston on its up stroke. The purpose of the stroke is to raise the temperature of the trapped air to a point where it will ignite the fuel charge once it is injected. Pressures of 35 bar and 540C are average values for the end of this cycle

Answer 15

The working stroke of this type of engine tends to be shorter than the typical 4 stroke engine. This is necessary to provide sufficient time to carry out the gas exchange process at the end of the stroke. As well, exhaust temperatures also tend to be lower due to the mixing of the relatively cool scavenge air with the exhaust gases.

Answer 16

- Principle bearings work under more severe stress - Require a blower or scavenge pump which may absorb 8-10% of the engine output - Double the number of power strokes in a given number of revolutions of the crankshaft so considerably more power output is obtained for equal weight and space - Each down stroke of the piston is a power stroke, therefore it gives a move even turning moment for the same number of cranks and a lighter flywheel can be used - Requires fewer cylinder - Simpler reversing arrangements since less valves are required for servicing the engine and thus less equipment is necessary to change timing - Can be constructed without air intake or exhaust valves, reducing maintenance costs and the complexity of the cylinder head

Answer 17

Used as the base of the engine and is a support for the main bearings. Can also be used to hold the lubricating oil

Answer 18

Generally located in the lower part of the engine, transforms the reciprocating motion of the pistons into rotary motion. Supported by the main bearings, also used to drive various pumps and the camshaft

Answer 19

Fitted in the bedplate and support and align the crankshaft. Covered with white metal (anti friction surface) to promote lubrication, reducing friction between fixed and moving surfaces and reducing wear of the crankshaft

Answer 20

The structure or skeleton of an engine and used to maintain or support the crosshead of a large bore diesel engine

Answer 21

Houses the cylinder where combustion takes place. They are cast with internal passages for cooling and lubrication. May be fitted directly on the A frame (for large diesel engines) or seated directly on the crankcase of bedplate (smaller diesel engines)

Answer 22

Inserted into the cylinder block. Forms the sides or walls of the combustion chamber and the piston slides vertically inside. Made of cast iron and are replaceable

Answer 23

Located inside the cylinders and the crown acts as the lowest part of the combustion chamber. Absorb the pressure of combustion and transmit it to the connecting rod (and in turn the crankshaft). May be cooled by water or oil.

Answer 24

Circular rings that are either square or rectangular in cross section. Fitted to the pistons and seal the combustion chamber from the crankcase (small engines) or prevent the combustion gases from leaking into the scavenge air manifold (large bore engines). May be designed to control the lubrication on the cylinder walls - called oil control rings

Answer 25

Fitted to the underside of the piston in large diesel engines. Does not articulate and is connected to the crosshead.

Answer 26

Fitted to a large bore engine, is responsible for transforming the vertical motion of the piston into the articulated movement of the connecting rod. The A frame will be fitted with guides which support the crosshead and allow it to move vertically.

Answer 27

Short shaft that forms the link between the piston and the connecting rod in trunk type engines

Answer 28

On trunk type engines: connects the piston to the crankshaft. Its movement is articulated around the wrist pin of the piston. On crosshead type engines: connects the crosshead and the crankshaft. Movement is articulated around the crosshead. The small end of the connecting rod is fitted with a bushing and the large end is fitted with bearing shells. These minimize friction and wear of all parts involved

Answer 29

Located at the top of the cylinder block and seals the combustion chamber. Also used to support the intake valves, the exhaust valves, the air start valves, and the fuel injectors. Cast with internal passages for cooling water and other passages for exhaust and air intake.

Answer 30

Inlet and exhaust valves (poppet valves) are fitted to the cylinder head and allow supply air and exhaust to enter and exit the cylinder. One or more inlet and exhaust valves may be fitted per cylinder.

Answer 31

Connected to the crankshaft by gears or a chain, and is times to match the angular movement of the crankshaft. In a 4-stroke engine, the camshaft is fitted with two lobes per cylinder (one for the intake valve and one for the exhaust) and these lobes convert rotary motion to linear motion, which in turn opens and closes the inlet and exhaust valves. A third lobe may be fitted which will operate a fuel pump for the cylinder

Answer 32

Series of interconnected gears or gears that are driven by chains or belts. Used to operate the camshaft as well as the pump drivers, which are necessary for proper operation of the engine.

Answer 33

Fitted into rollers, called cam followers, follow the profile of the camshaft lobes. These rollers actually transform the rotary movement of the cams into vertical movement. This vertical movement is transmitted to the rocker gear (valve operating mechanism) which opens and closes the valves. On small diesel engines, there are no push rods; the valves are actuated directly by the camshaft.

Answer 34

Provide the injector with a precisely metered volume of high pressure fuel. Times off the camshaft and deliver fuel at the proper point in the engine cycle to ensure complete combustion at a given speed or load. May be ganged together (more than one fitted) and drive independently off the camshaft OR may be individual to each cylinder and activated by a lobe on the camshaft

Answer 35

Used to introduce atomized fuel into the cylinders. May be activated hydraulically or electronically.

M1: Internal Combustion Engines Flashcards

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