- #1

skrelt

- 2

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**2 Questions -- Heating room air, and fuel combustion efficiency**

## Homework Statement

2 questions. In advance, i needed to translate it so i am sorry if it is states incorrect. I really did my best to do it right.

1.

The density of air at 1 atm and 10.0 ° C is equal to 1.24 kg / m3. (cubic meter)

A room has the following sizes: 8,0 m x 4,5 m x 2,7 m

Calculate how long it takes for a heater of 1.5 kW to heat the air from the room to 21.0 ° C.

Use c = 1005 J / kg / K

2.

An internal combustion engine uses a fuel with calorific value/heating value of 38 MJ / kg.

The density of the fuel is 900 kg / m3. (cubic meter)

Of each MJ heat only 0.25 MJ is converted to mechanical energy.

The efficiency is 25%.

The engine can deliver 30 kW.

Calculate how many liters of fuel per hour the engine used.

## Homework Equations

http://imgur.com/WQx3BqI

these are the once i used. I probly made some/alot mistakes so i don't know if i need more.

## The Attempt at a Solution

1.

8,0m x 4,5m x 2,7m = 97,2m3 (cubic meters)

1 atm and 10.0 ° C = 1,24 kg/m3

Heater 1,5 kW = 1500 W = 1500 J/s

C = 1005 J/kg/K

This is where I am atm. I figured I needed to change the heater to J/s hence you need to know how long it takes. But after I gathered all the data I could find I don’t know where to start with the calculation or what formula to use first.

2.

Usage = 38 MJ/kg = 0,38 x 10^8 J/kg

P = 900 kg/m3 (cubic meters)

Energy usage = x 0.25

30 kw = 30000 J/s

30000 J/s x 3600 = 1,08 x 10^8 J/hour (first I figured I needed to set it in hours)

0,38x10^8 / 1,08x10^8 = 0,3519 J/kg/hour

900x0,3519 = 316,71 m3 (cubic meters)

316,71 m3 = 316710 dm3 = 316710 Liters of fuel.

I doubt there is an engine that uses that much so I think I made some mistakes.

I hope i can get some help here on 1 or maybe even both questions. Thanks in advance. I tried my best on both and these are the answers and solutions i could produce