Air density effects on fuel efficiency/power

[boeingwest]

Hi. One of the younger lads who i'm learning to fly with (we are in the very early pre GFPT stages) asked me at the aero club the other day in conversation, why it is that a plane burns less fuel and is less efficient (in power). I understand its to do with air density, but does anyone have a simple response. This goes for both piston and turboprop/fan engines.

Thank you in advance

[sms777]

OK. To put it simply a naturally aspirated piston engine is limited to the amount of air and fuel it can draw into itself naturally. At sea level due to denser air it will make more power than on top of a 6000 feet mountain due to less O2 available.
To overcome this setback we decided to bolt turbos and superchargers onto them and force more air and fuel in to combustion. More air and fuel= more power.
Someone else can explain the jet aspects because i have to go to fly a jet tonight ;)

[aditya104]

density=mass / volume
when qnh decreases. pressure decreases, air density decreases.
that means for a given volume of air, the mass of air decreases. i.e. the air becomes lighter, no. of molecules decrease.

on such a day, less no. of molecules will draw less amount of fuel. so the total charge(air+fuel) into the combustion cylinder will be less. this will result in less engine output and thus the engine power decreases.

Also, less air molecules affect the aircraft aerodynamically. Lift produced is less on a day when the density is less.

Aditya

[tmpffisch]

Keeping it simple, the less dense air is, the less air molecules there are.

There are less air molecules that can be mixed with fuel and burned and less air molecules available to create lift.

[The Green Goblin]

Very easy,

Increase Air density, Increase volume available for combustion.

Jet engines are a little more complex, they love cold dense air (especially a garrett) but the higher you go and the faster you go the greater the ram effects of the airflow and the better the fuel/air ratio giving a greater specific fuel consumption.

Ever wondered why the 'ole commonhoare goes better on a winters night than a hot summer day?? cooler denser air :)

[tio540]

Increase Air density, Increase volume available for combustion.


The volume of air in the powerplant doesn't change. The mass changes with the density of the air.

Altitude lowers mass of air in the engine, but we can increase the mass with turbochargers. We can also increase the mass of air at sea level with turbochargers.

Interestingly the volumetric efficiency of the powerplant increases with altitude because the back pressure reduces.

Hope that helps.

[Mark1234]

My goodness, make a simple thing complicated(!)

Lower air density = fewer O2 molecules (being pedantically correct)

Burning as a chemical process is oxidisation - combining the fuel with oxygen (O2) So, if there are fewer O2 molecules, you can burn less fuel. Deliver as much as you like, what counts is the ability to burn it, and so release energy. That's it, plain and simple.

The double whammy is that your reaction mass is also air, so it doesn't end with reduced power output from the engine. The prop is less efficient, and the volume of air it shifts has less mass, so thrust is reduced. The and the wing requires more airflow, or a higher angle of attack (more drag) to generate the same lift.

It's all downhill.....

[nomorecatering]

Doesnt range decrease in a piston engine as with colde, more dense air, the fuel flow is higher?

[Oktas8]

Fuel flow is proportional to power in a correctly leaned piston engine, regardless of air density. In some respects flying higher is better (airframe drag for a given TAS), in some respects flying low is better (saves fuel otherwise used for climbing).

As to range reducing or increasing with altitude, keep it simple. Look in the flight manual, Section 5. There you will find a table (or graph) of maximum range against altitude and power setting. Easy answer!

Cheers,
O8