Engine Power and Forced Induction Systems

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Questions and Answers

What is a major reason for the poor fuel economy of supercharged petrol engines?

Reduced sensitivity to fuel quality

Increased pressure and temperature of intake air

Lower compression ratios and intercooling

Higher specific heat values and dissociation losses (correct)

How does supercharging affect the combustion in diesel engines compared to petrol engines?

It decreases combustion efficiency.

It makes diesel engines more sensitive to fuel type.

It improves combustion by reducing ignition delay. (correct)

It leads to higher combustion temperatures.

What is the impact of increased flame speeds in supercharged petrol engines?

It requires richer mixtures to prevent knocking. (correct)

It results in lower specific fuel consumption.

It enhances the overall thermal efficiency.

It allows for more flexible fuel-air mixtures.

What is one method used to control knocking in highly supercharged petrol engines?

Injecting water into the combustion chamber Signup and view all the answers

Why might intercooling be unnecessary for most supercharged diesel engines?

The increase in pressure compensates for the reduction in efficiency. Signup and view all the answers

What affects the life of exhaust valves in supercharged engines?

Lower fuel-air ratios and high expansion ratios Signup and view all the answers

What is the reason for increased specific fuel consumption in supercharged petrol engines?

Control measures to prevent detonation Signup and view all the answers

Which of the following is a drawback of using a supercharged petrol engine?

Higher fuel consumption under most conditions Signup and view all the answers

What is one main benefit of supercharging an engine?

Increases the amount of air inducted for the same swept volume. Signup and view all the answers

Which statement is true regarding the pressure in supercharged engines compared to unsupercharged engines?

There is an increase in pressure over the unsupercharged cycle. Signup and view all the answers

What happens to the mechanical efficiency of an engine when supercharging is applied?

Increases. Signup and view all the answers

Why is supercharging particularly utilized in aircraft and racing car engines?

It enhances volumetric efficiency while controlling detonation. Signup and view all the answers

How is the power required for driving the supercharger derived?

By writing down the steady flow energy equation. Signup and view all the answers

What is the role of the pumping loop in a supercharged engine?

It is positive rather than negative. Signup and view all the answers

What effect does increased intake pressure have on ignition delay and flame speed?

It reduces ignition delay and increases flame speed. Signup and view all the answers

What is the primary purpose of a supercharger in an engine?

To increase the power output Signup and view all the answers

What assumption is made when deriving the work needed by the supercharger?

The heat loss from the supercharger is disregarded. Signup and view all the answers

Which factor does NOT affect the power output of an engine?

Cylinder wall thickness Signup and view all the answers

How can the amount of air inducted per unit time be increased?

By increasing the engine speed Signup and view all the answers

Why is supercharging preferable over increasing the compression ratio for power output?

Supercharging provides more power with less increase in temperature Signup and view all the answers

What is a significant objective of supercharging in aircraft engines?

To compensate for power loss due to altitude Signup and view all the answers

What is a consequence of increasing the engine speed for power output?

Increased engine friction and bearing loads Signup and view all the answers

Which of the following best describes the relationship between supercharging and break mean effective pressure?

Supercharging increases break mean effective pressure more effectively than just increasing compression ratio Signup and view all the answers

What is an important consideration when supercharging for aviation engines?

Minimizing altitude effects on power Signup and view all the answers

How does supercharging affect the power output of an engine compared to a naturally aspirated engine?

It increases power output significantly. Signup and view all the answers

What factor limits the degree of supercharging in an engine?

Thermal and mechanical loads Signup and view all the answers

Why do supercharged petrol engines tend to knock more than diesel engines?

Use of richer fuel mixtures Signup and view all the answers

What is a drawback of mechanically driven superchargers at part loads?

Greater loss in power utilization Signup and view all the answers

Which aspect does NOT contribute to increased specific fuel consumption in mechanically driven Otto engines?

Higher exhaust gas temperatures Signup and view all the answers

What is the specific fuel consumption of CI engines compared to naturally aspirated engines?

Somewhat less than naturally aspirated engines Signup and view all the answers

Which of the following factors does NOT limit the power output of a supercharged engine?

Fuel injection system design Signup and view all the answers

Study Notes

Forced Induction Systems

Forced induction systems increase the inlet air density, thus increasing the mass of air entering the engine during the suction stroke. This is called supercharging.
Naturally aspirated systems rely on pressure differences between the cylinder and atmosphere during the suction stroke.
Supercharging boosts engine power output by supplying air at a higher pressure than that created during natural suction. This is achieved using a supercharger, a pressure-boosting device.

How to Increase Engine Power

Engine power depends on the amount of air intake per unit time, how efficiently that air is utilized, and the thermal efficiency of the engine.
Increasing engine speed or increasing the density of intake air can increase the amount of air inducted per unit time.
Higher engine speeds lead to greater inertia loads, requiring a rigid, robust engine design.
Higher speeds also increase friction and bearing loads, and decrease volumetric efficiency.
Increasing the compression ratio increases maximum cylinder pressure. The increase in break mean effective pressure due to a supercharger is greater than that due to increasing compression ratio for a given maximum cylinder pressure.
Supercharging results in lower rates of maximum temperature increase, which in turn leads to lower thermal loads.

Objectives of Supercharging

Supercharging increases the amount of air inducted per unit time, enabling the engine to burn more fuel and thus increase power output.
Supercharging objectives include:
- Increasing engine power output for a given weight and bulk.
- Compensating for power loss at high altitudes.
- Obtaining enhanced power from a pre-existing engine.

How Supercharging Works

Supercharging increases intake air pressure above atmospheric pressure.
Pumping loop in supercharged engines is positive instead of negative.
The increased inducted air at higher density leads to increased output.

Supercharging Power Calculation (Steady Flow Process)

Power required to run a supercharger is calculated through the steady flow energy equation.
The equation considers the air's internal energy and pressure changes during the intake and exhaust processes.
Assuming zero heat loss from the supercharger simplifies the equation (W=(E2+P2V2)-(E1+P1V1)) and is further simplified to W = h2-h1 = cp(T2-T1)
T2 is the temperature at the end of compression in the supercharger,
cp is specific heat at constant pressure,
T1 is the initial intake air temperature and

Supercharging of Spark-Ignition Engines

Supercharging is typically only used in aircraft and racing engines due to the increased tendency for detonation and pre-ignition.
Increased intake pressure and temperature decrease ignition delay, and increase flame speed, both increasing the likelihood of detonation.
Lower compression ratios are used in supercharged spark-ignition engines to help mitigate these issues, leading to decreased thermal efficiency.
Increased fuel consumption is an outcome.
Rich mixture control or intercooling can lessen this, but added liquid handling poses a challenge.

Supercharging of Compression-Ignition Engines

Supercharging improves combustion quality in diesel engines, reducing ignition delay and resulting in smoother combustion.
Supercharging permits use of poorer quality fuels.
Increased intake air density contributes to larger volumetric and thermal efficiency increases.
Supercharging doesn't necessitate intercooling in most instances, except for highly supercharged engines.
Mechanical and thermal loads increase with supercharging but are generally moderate due to lower compression ratios, valve design, cooling effects, and improved combustion.
Lower fuel-air ratios may be employed, resulting in lower temperatures over the engine cycle.
Supercharging leads to improved engine reliability, durability, and exhaust valve life due to smoother combustion and reduced exhaust gas temperatures.

Effect of Supercharging on Engine Performance

Supercharging increases power output due to:
- Increased air intake per cycle.
- Slightly improved mechanical efficiency
- Reduced exhaust gas temperatures.

Supercharging Limits

Power output is limited by knock, thermal, or mechanical stresses, depending on the engine type.
Knock limit is reached first in spark-ignition engines, and thermal/mechanical limits first in compression-ignition engines.
The degree of supercharging on spark-ignition engines is limited by knock and the temperature of piston/cylinder.
Supercharging may cause scuffing of piston rings and heavy liner wear if temperatures are too high.

Methods of Supercharging

Various diagrams or visual aids display schematic representations of supercharger systems.
Diagrams and/or visuals are used to describe different methods of supercharging like direct-drive or belt-drive systems.

Fuel Consumption

Supercharger power demand affects fuel consumption.
Mechcnaically driven supercharger consumes more fuel at part loads.
Fuel consumption is often higher in supercharged spark-ignition engines due to very rich mixtures required to mitigate knocking issues.

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Description

Explore the role of forced induction systems like supercharging in enhancing engine power output. Understand the differences between supercharged and naturally aspirated systems, and learn how air intake and engine speed affect power efficiency. This quiz will help consolidate your knowledge of engine dynamics and performance enhancements.