Passage:Internal Combustion Engine

From MyMCAT

Jump to: navigation, search

The internal combustion engine is an engine in which the combustion of fuel and an oxidizer (typically air) occurs in a confined space called a combustion chamber. This exothermic reaction creates gases at high temperature and pressure, which are permitted to expand. The defining feature of an internal combustion engine is that useful work is performed by the expanding hot gases acting directly to cause movement of solid parts of the engine, by acting on pistons, rotors, or even by pressing on and moving the entire engine itself.


This contrasts with external combustion engines, such as steam engines and Stirling engines, which use an external combustion chamber to heat a separate working fluid, which then in turn does work, for example by moving a piston or a turbine.


All internal combustion engines must achieve ignition in their cylinders to create combustion. Typically engines use either a spark ignition (SI) method or a compression ignition (CI) system. In the past, other methods using hot tubes or flames have been used.


image:Passage_int_comb_eng_PVdiagram.jpg


A typical combustion cycle can be broken into six numbered stages based on the mechanical operation of the engine, these six stages are labeled in figure one. The cycle begins at the lower left with Stage 1 being the beginning of the intake stroke of the engine. The pressure is near atmospheric pressure and the gas volume is at a minimum. Between Stage 1 and Stage 2 the piston is moved to the left, the pressure remains constant, and the gas volume increases as fuel/air mixture is drawn into the cylinder through the intake valve. Stage 2 begins the compression stroke of the engine with the closing of the intake valve. Between Stage 2 and Stage 3, the piston moves back to the right, the gas volume decreases, and the pressure increases because work is done on the gas by the piston. Stage 3 is the beginning of the combustion of the fuel/air mixture. Stage 4 begins the power stroke of the engine. Between Stage 4 and Stage 5, the piston moves back to the left, the volume in increased, and the pressure falls. At Stage 5 the exhaust valve (blue) is opened and the residual heat in the gas is exchanged with the surroundings. The volume remains constant and the pressure adjusts back to atmospheric conditions. Stage 6 begins the exhaust stroke of the engine during which the piston moves back to the right, the volume decreases and the pressure remains constant. At the end of the exhaust stroke, conditions have returned to Stage 1 and the process repeats itself.


The work times the rate of the cycle (cycles per second) is equal to the power produced by the engine. In reality, each cycle is not ideal and there are many losses associated with each process. These losses are normally accounted for by efficiency factors which multiply and modify the ideal result. For a real cycle, the shape of the p-V diagram is similar to the ideal, but work is always less than the ideal value.


1. Assuming an ideal engine, what would be the approximate power output of the engine described in figure 1 be if it were running at 100 rpm (revolutions per minute)?

9500KW.
19000KW.
→ The passage states that power is given by the work times the rate (in seconds). To calculate the work, one must calculate the area of the cycle in the PV diagram. This can be approximated as about 32. (Cut the shape into two triangles one with area 8, the other with area 24.) This work in atm*L should be converted to joules (~101J /atm*L and then should be multiplied by the number of cycles per minute (100) and followed by 60 (to convert the minutes to seconds). Assuming all these calculations are performed correctly, 19392000 J is the answer. We must then realize that we have made an overestimate in area, and 19000KW is the most likely answer.
32500 KW.
65000 KW.

2. During which stage or stages is work done by the gas?

Between stages 1 and 2.
Between stages 3 and 4.
Between stages 4 and 5.
Between stages 5 and 6.
→ Stated in the passage, combustion begins at stage 3. Work however is not performed by the gas from stages 3 to 4 because only the pressure increases. It is only during stage 4 to 5 is the gas is allowed to expand, performing work on the piston.

3. If the temperature of an ideal monatomic gas is quadrupled, what is the effect on the gas' internal energy?

The energy does not change.
The energy is quadrupled.
→ The equation for total internal energy of a gas is U = (3/2)nRT. Thus temperature and energy are directly related.
The energy is doubled.
The energy is halved.

4. If 1400J were absorbed by the 1.2kg carbon piston during the combustion stage, by how many degrees would the piston's temperature rise? (heat capacity of graphite carbon: 0.72 J0K-1g-1)

19.4 oC
→ Using q = m*c* change in T, we find that T = q/(m*c). Remembering to change kg to grams and then to convert to moles carbon, we find: T = (1400)/((.72)( 1.2kg * 1000g/kg / 12 g/mol)) = 19.44. The math in this question is much simpler if one uses .7 instead of .72, then 1400/.7 = 2000.
26.2 oC
38.8 oC
52.4 oC

5. An adiabatic process is one in which?

Temperature remains constant in the system.
→ An isothermal process is one in which temperature is not allowed to change. But it is possible for this to occur through heat transfers along with compressions or expansions.
The pressure is held constant.
→ When pressure is held constant it is called isobaric.
The total energy of the system is held constant.
No heat is transfered to or from the system.
→ In an adiabatic process heat is not able to move to or from the system, thus the temperature can easily change and depending on if work is done by compression or expansion, the total energy may not stay the same.

6. What is the change in heat of a system which loses 600J while expanding to twice its original volume of 100L at a constant pressure of 2atm?

-400J
→ recognizing that the change in U is 600J and that the work, W = Pressure * change in Volume, we get: Q = (Change in U) + P*(Change in Volume) = (-600J)-(2 * 100) = -400.
+400J
-800J
800J

Your score is 0 / 0
Retrieved from "http://mymcat.com/wiki/Passage:Internal_Combustion_Engine"
Views
Personal tools