Otto Cycle Processes and Efficiency

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Questions and Answers

What does BMEP stand for in the context of gasoline engines?

Base Mean Efficiency Point

Brake Mean Effective Pressure (correct)

Brake Mass Energy Production

Boosted Mean Energy Performance

Which of the following accurately describes η_brake?

It is the ratio of the power generated to the total engine displacement.

It represents the thermal energy lost during combustion.

It directly indicates the maximum RPM achievable by the engine.

It measures the brake power output relative to fuel energy input. (correct)

At what RPM is the engine performance data provided?

2000 RPM (correct)

4000 RPM

1000 RPM

3000 RPM

What does CR stand for in the context of engine specifications?

Compression Ratio Signup and view all the answers

Which fuel charge type is not mentioned in the context provided?

Gasoline Direct Injection (GDI) Signup and view all the answers

What is the primary characteristic of the heat addition process in the Otto cycle?

Heat is added at constant volume. Signup and view all the answers

Which process in the Otto cycle is responsible for the conversion of thermal energy to work?

Isentropic expansion Signup and view all the answers

In the Otto cycle, which relationship correctly defines the cycle efficiency?

ηcycle = 1 - k - 1 / T2 Signup and view all the answers

During which phase does the intake/compression take place in the Otto cycle?

1 to 2 Signup and view all the answers

What does 2Q3 represent in the Otto cycle equation?

Change in internal energy Signup and view all the answers

Which statement about specific heats (Cp, Cv) in the Otto cycle is true?

Specific heats are constant. Signup and view all the answers

What is the primary purpose of the constant volume heat removal in the Otto cycle?

To prepare for the next power stroke. Signup and view all the answers

What type of process occurs between states 3 and 4 in the Otto cycle?

Isentropic expansion Signup and view all the answers

Which statement best describes the differences in efficiency between SI and CI engines?

The efficiency comparison varies based on fuel type and operational processes. Signup and view all the answers

What is the typical compression ratio (CR) range for SI gasoline engines?

10:1 to 12:1 Signup and view all the answers

How is load controlled in a CI engine compared to an SI engine?

By adjusting the fuel injection pulse width in CI engines. Signup and view all the answers

Which factor most significantly contributes to throttling losses in SI engines?

Throttling air, which takes work. Signup and view all the answers

What is the consequence of retarding spark timing in an SI engine?

It increases the combustion duration. Signup and view all the answers

What technique can improve the efficiency of ICEs?

Implementing turbocharging technologies. Signup and view all the answers

What is considered a limitation in the efficiency of gasoline engines?

Structural materials limiting peak pressures. Signup and view all the answers

What is a common misconception about the power delivery of an Otto cycle compared to a Diesel cycle?

An Otto cycle can deliver more power depending on conditions. Signup and view all the answers

What is one advantage of gasoline direct injection (GDI) technology?

Improved fuel efficiency by more than 15% Signup and view all the answers

Which automakers were among the first to develop GDI technologies?

Honda, Toyota, Nissan, and Mitsubishi Signup and view all the answers

Regarding market penetration, what factor significantly affects U.S. vehicle ownership?

The average duration of vehicle ownership Signup and view all the answers

What features characterize a stratified charge in gasoline direct injection?

Partial premixing of fuel and air Signup and view all the answers

What was a key feature of the GDI Ecoboost technology introduced by Ford in 2009?

Turbocharged engines delivering 20-30% improvement in fuel economy Signup and view all the answers

What is the size and design characteristic of the single-cylinder optical engine detailed?

Based on a flat piston with a specific window design Signup and view all the answers

Which statement accurately describes the ownership behavior of U.S. vehicle owners?

They tend to keep SUVs longer than sedans. Signup and view all the answers

What does the abbreviation 'P.C.F.I.' stand for in the context of engine technology?

Port Fuel Injection Signup and view all the answers

What is the main reason diesel cycles are generally more efficient than Otto cycles?

They can use higher compression ratios. Signup and view all the answers

Which of the following formulas represents the thermodynamic efficiency according to the first law of thermodynamics?

$\eta_{1st} = \frac{W_{cycle}}{Q_{in}}$ Signup and view all the answers

What is the primary factor contributing to fuel economy expressed in MPG?

The mass of fuel consumed. Signup and view all the answers

Which statement about engine and drivetrain efficiency is correct?

Total vehicle efficiency is the product of engine and drivetrain efficiencies. Signup and view all the answers

What does the term 'Wprop' represent in the context of vehicle propulsion?

The actual work required to overcome drag and friction. Signup and view all the answers

What is the effect of engine losses on vehicle energy distribution?

They contribute significantly to energy loss. Signup and view all the answers

In calculating the vehicle's MPG, which variable directly relates to the efficiency of the engine and drivetrain?

The total energy consumed from fuel. Signup and view all the answers

Which formula represents practical efficiency in terms of mass of fuel and the heating value of the fuel?

$\eta_f = \frac{W_{actual}}{m_f Q_{HV}}$ Signup and view all the answers

What does the term 'CD' refer to in the context of vehicle aerodynamics?

Coefficient of drag. Signup and view all the answers

Which of the following is NOT a component of the total energy loss in a vehicle?

Passenger comfort Signup and view all the answers

Study Notes

Otto Cycle: A Cycle of Four Processes

Consists of four processes: Isentropic Compression, Constant Volume Heat Addition, Isentropic Expansion, and Constant Volume Heat Removal
Isentropic compression (intake/compression stroke)
Constant volume heat addition (combustion)
Isentropic expansion (power stroke)
Constant volume heat removal (exhaust stroke)

Otto Cycle Efficiency

Thermal efficiency (ηcycle) is a function of compression ratio (CR) and the specific heat ratio (k)
The higher the compression ratio, the higher the efficiency
The higher the specific heat ratio, the higher the efficiency

Otto vs. Diesel

Otto cycle (constant volume heat addition) delivers more power than Diesel cycle (constant pressure heat addition)
Gasoline engine is constrained by fuel properties = lower CR
Typical CR:
- SI gasoline CR = 10:1 to 12:1
- DI diesel CR = 15:1 to 20:1
Materials are limited by peak pressures and maximum rates of pressure rise

Real Engine P-v Diagram

SI engine: both fuel and air are throttled to control load (power)
CI engine: load is controlled by fuel injection pulse width
Throttling air takes work – up to 20% of power out
Area represents net work required; eliminate pumping work to increase efficiency

Chemiluminescence during Otto Cycle Combustion

Piston view imaging, indolene fuel
Retarding spark timing
Duration of combustion
Start of combustion

Improving ICE Efficiency

HCCI-TC, LEAN-SI-TC, ADV-TC (TC = turbocharging, NA = naturally aspirated (unboosted))
BMEP = brake mean effective pressure (power)

Efficiency Definitions

Thermodynamic Efficiency:
- 1st Law: η1st = (Wcycle)/(Qin)
- 2nd Law: η2nd = (Wactual)/(Wmax) = (Wactual)/(Wisentropic)
Practical Efficiencies:
- Based on the energy content of the fuel and work out of engine: ηf = (Wactual)/(m f ⋅ QHV)
- Based on power required to propel the vehicle: ηp = ( W prop )/(m f ⋅ QHV)

Engine and Drivetrain Efficiency

Engine Efficiency (ηengine) is affected by heat losses, friction losses, and throttling losses
Drivetrain Efficiency (η DT ) is approximately 80%
Overall efficiency (ηtotal) is roughly 15%

Fuel Economy and Engine Efficiency

Fuel economy is measured in MPG (Miles per Gallon)
MPG is directly proportional to ηtotal
MPG is influenced by factors like drag, rolling friction, and engine efficiency

Distribution of Energy in Vehicle

Engine losses are the largest source of energy loss

Supplemental Materials

Otto Cycle Process
Easley at el., DEER 2005
Single-cylinder optical engine research facility [Sandia]
Gasoline direct injection (GDI)

Production GDI

Early adopters: Japanese automakers (Honda, Toyota, Nissan, Mitsubishi)
Ford introduced Ecoboost technology in 2009
GM offered 6 GDI engines in 2009
Hyundai (announced 2009)

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Description

Explore the four fundamental processes of the Otto cycle, including isentropic compression, constant volume heat addition, isentropic expansion, and constant volume heat removal. Understand how thermal efficiency is influenced by compression ratio and specific heat ratio, and compare Otto cycle efficiency to Diesel cycle performance.