Engine Efficiency: 1.4 kg Gasoline/hr = 3.5 kW Power

[chawki]

Homework Statement

An engine burns 1.4 kg of gasoline per hour and yields 3.5 kW of power.

Homework Equations

Calculate the efficiency of the engine when the heating value of gasoline is 43 MJ/kg.

The Attempt at a Solution

Plz i just need the rule for engine efficiency.

 

[GuitarStrings]

Homework Statement

An engine burns 1.4 kg of gasoline per hour and yields 3.5 kW of power.

Homework Equations

Calculate the efficiency of the engine when the heating value of gasoline is 43 MJ/kg.

The Attempt at a Solution

Plz i just need the rule for engine efficiency.

Coal burnt per second (A) = \frac{1.4}{60 * 60}
Power through coal burnt per second (P) = (43 * A)
P x Efficiency of engine (e) = 3.5kW

Find e. Be careful with the units and the conversion.

 

[chawki]

ok so:

efficiency = power generated/heat energy ?
and then:
power generated= 3.5kw
heat energy=m_fuel*heating value

consumption per hour =m_fuel*3600
m_fuel=1.4/3600=3.88*10e^-4 Kg/s

heat energy: Q= 3.88*10e^-4*43=16.684kw

Efficiency= 3.5/16.684 = 0.209

 

[GuitarStrings]

ok so:

efficiency = power generated/heat energy ?

Not really.

Power of Engine = Efficiency of engine x Energy density of coal x Coal burnt per second

 

[chawki]

we would find same results, no?

 

[GuitarStrings]

I'm not sure, I didnt read beyond the first line because I assumed the rest of your post built upon that formula, which was wrong.

Because Efficiency = Power (W) /Heat Generated (J)
So it the unit for efficiency would be s^-1.

Efficiency has no unit.

 

[chawki]

in my work, efficiency has no unit.
it's kw/kw

 

[Andrew Mason]

in my work, efficiency has no unit.
it's kw/kw

You are correct. Efficiency is the ratio of energy output to energy input. This is the same as power output/power input.

AM

 

[chawki]

You are correct. Efficiency is the ratio of energy output to energy input. This is the same as power output/power input.

AM

Should we leave 0.209 as it is, or should we write 0.209*100% = 20.9% ?