why piston engine avgas and turbine engine avtur? 2

[vasanx]

hi there!

just wanted to know why does piston engine use gasoline and turbine engine use aviation turbine fuel?

 

[jsummerfield]

Not all piston engines use gasoline. Diesel engines have pistons and use Diesel fuel oil. The new aviation Diesel engines use the kerosine based jet fuel oil like Jet A1.

As a wild guess, perhaps the internal combustion engines during the days of the Wright brothers were run on drip-gas. Drip-gas is the condensate associated with gas wells.

John

 

[Compositepro]

The two types of engines operate differently and have different requirements for their fuel. Octane rating or resistance to preignition (pinging) is important in piston engines where the comsustion occurs in pulses. Increasing octane rating increases cost and is not needed for jets.In piston engines the fuel must get vaporized, mixed with air and ignited each cycle.

In jets the fuel is injected into the flame front which burns continuously. The fuel can be less volatile (higher flashpoint) so the safety in handling and storing fuel can be increased. Diesels are often the choice for marine engines because fuel leaks are much less of a hazard.

 

[vasanx]

thanks for the info.

but why is octane rating not important in turbine engine?
shouldn`t there be an equivalent factor for avtur since both gasoline and kerosene are derived from the source?

 

[Compositepro]

Octane rating is not relavent to jet engines because jets don't ping (except maybe in the movie "Airplane&quot. It is a fuel characteristic that is only important to higher compression ratio piston engines. Using a higher octane rated fuel than is needed for an engine does nothing except waste money. An engine either pings or knocks or it doesn't. If it doesn't knock the octane rating is adequate and additional octane does nothing.

Knocking is caused by the fuel in the cylinder igniting before the spark plug fires due to the heat of compression.

 

[vasanx]

thanks again compositepro. really appreciate your info.

but please bear with me just yet.

1. if octane rating is not important in jet engines then why do
they say don`t put avgas into a turbine engine. it`s not like
the engine is going to knock right?
2. why should only avtur be used specifically for turbine
engine.
3. is it because of cost?
4. is gasoline more expensive than kerosene?
5. or is the production of gasoline would never be able to
meet the demands of current turbine engines seeing as
how many they are in the market now an so that`s why
avgas is for piston engined aircraft and kerosene for the
former?

 

[RobWard]

I seem to remember being told that the Harrier can use pretty much any fuel available.

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[jsummerfield]

Vasanx ask several questions that deserve an answer even if not by one with expertise.

Octane rating pertains to antiknock or pre-ignition characteristics. We already mentioned that piston engines include Diesel. After Diesel and forgetting about Wankle engines, most remaining internal combustion engines gasoline powered engines ignite the fuel by means of a spark. The higher octane rating is associated with higher compression ratio. Higher compression results in more heat and also tends to ignite the gasoline before introducing the spark. This is not a good thing when you are trying to control the ignition timing.

Don’t put avgas into a turbine engine because it is setup to burn Jet A1, a kerosene product. Gasoline burns too easily for these engines. I think that jet engines can be setup to burn gasoline. Other fuels such as JP4 were used in earlier jets. As another post said, gasoline and JP4 are more volatile and pose a safety hazard.

Don’t put gasoline into Diesel engines for the reason that the engine would ignite the fuel much too soon, perhaps run away, etc. However, the turbine engine is actually very flexible regarding the fuel. A jet engine can be setup to run on many different types of fuel but only one type at a time. On this offshore platform we run GE LM2500 aircraft derivative engines on fuel gas, an off-product of natural gas processing. The turbine manufacturer needs the fuel specification to set up the engine and the selected fuel needs to maintain similar characteristics all of the time.

I know noting of Avtur. Check the ASTM or API for specific jet fuels. Jet A1 is the fuel that is commonly used for domestic turbine engines.

The marketing of gasoline, Diesel and kerosene balance supply and demand. Gasoline is generally more expensive than Diesel and likely kerosene too. I have not checked. Crude oil distillation includes a wide range of products with gasoline on the light end and asphalt on the heavy end. Kerosene is still used as number 1 fuel oil in some parts of the US. Gasoline production barely keeps up during the summer months.

Most US refineries are old with little new investment as there is little profit. There are few new refinery projects. These projects often address legislation like low-sulfur Diesel production. Previous legislation required oxygenated reformulated gasoline for big cities. The resulting MTBE additive now results in litigation as the distributors had leaks in storage tanks and MTBE was found in ground water.

Gasoline is a very dangerous product. Besides the safety issue, gasoline would not likely meet the demands of current turbine engines even if redesigned to be suitable and the cost would be prohibitive for the aviation energy.


John

 

[vasanx]

thank you so very much for your postings, it is indeed very enlightning.

thank you again.

 

[avio]

Hello Vasanx,
check following website and you will learn a lot on Piston engines, running on Jet A or Jet A1 fuel! Jsummerfield already mentionned it above.

http://www.smaengines.com

They are FAA and JAA approved and have an STC for the Cessna 182. Other STC's and TC's are in development.
Rgds, Avio

 

[PEW]

(A few thoughts from a pilot).

AVTUR is AViation TURbine fuel, a military specification, very similar to Jet-A1. AVTUR contains an icing inhibitor, Jet A-1 might not.

Gasoline has a higher vapour pressure than Diesel or Kero based fuels. At the sort of altitudes that jets go (generally much higher than most piston engined aircraft), the possibility of vapour lock becomes an issue with gasoline.

Pinging / knocking before a spark gets involved is more commonly known as pre-ignition or auto-ignition / dieselling in UK. Pinging can also occur AFTER spark ignition has taken place, due to a too-rapid build up of combustion pressure causing the firing off (detonating) of the remaining fuel / air mixture ahead of the flame front in the combustion chamber. This is why gasoline fuelled piston engines have manufacturer's limits on the manifold air pressure at low RPMs.

The critical limit on most jet engines is Turbine Gas Temperature (TGT). Running an inappropriate fuel may cause the TGT limits to be reached before maximum engine RPM is attained, cutting down the achievable thrust of the engine. Most engines have a engine control system that prevents the TGT limit being exceeded. The aircraft manufacturer's published performance data requires the engine to push out it's full rated power every time. A high TGT would therefore quite possibly take the aircraft outside the limits for safe takeoff performance, for example, i.e. it might finish it's take-off roll in a field rather than in the air.

 

[mbga9pgf]

I am guessing but I would imagine the Engine electronic controller in turbines compares Turbine entry temperature against mass flow of fuel to air. The heat of combustion generated per unit mass of avgas far exceeds that of avtur. furthermore, it is far more expensive, due to the fraction it is taken from in distillation. Although it is possible to run turbines on avgas, motor gasolene, even wood powder, they have to be specifically designed to do so. An excellent example of such is the Abrams A-1 tank (american MBT) which can operate off varying fuels depending upon a simple switch selection!
When it comes down to your original question however, it comes down to cost. a gas turbine could be designed to run off avgas, however, the relative cost of operation duce to fuel price would be prohibative. and in the aerospace industry, especially public transport, money is everything!

 

[Michaelah]

Some Turbine engines are rated for Avgas. An example is the Honeywell/Garrett TPE331 series. The primary limiting factor is the vapor pressure of the fuel. The TPE331-12JR requires that the fuel pressure at the inlet to the engine driven fuel pump be at least 5 PSI above the vapor pressure of the fuel to prevent pump cavitation. Additionally, this series engine has a maximum number of hours that you can run Avgas, if memory serves, it is about 100 Hrs. The primary reason for certifying the installation for Avgas, is the need for an alternate fuel. Some locations, all be it very few, don't have turbine fuel available.

 

[CFCady]

Aviation Gasoline Refining: Aviation FTR-3, Chapter 11, Chevron.
Avgas blending: [Aviation FTR-3, Chevron].

Aviation gasoline (avgas) is a highly refined product specifically manufactured to meet the demanding performance requirements of aircraft engines. Avgas specifications make it difficult to meet all the requirements with a single refinery stream, even one such as light alkylate produced specially for avgas. Avgas, like motor gasoline (mogas) and most other refinery products, is usually produced by blending two or more components to achieve the desired properties. [FTR-3, p.65, Chevron].
In 1930, the US Army Air Corps specified a ‘Fighting Grade’ avgas with a minimum octane number of 87. This is believed to be the first instance in which the antiknock properties of avgas were defined in terms of octane numbers. By the start of WWII, avgas very similar to today’s Grade 100 were in use. Avgas reached its development peak during WWII and the following decade. In 1944, the US military issued a specification for Grade 115/145. This fuel, which had the highest antiknock rating (performance number) of any avgas in large-scale production, was used to obtain maximum output from military high-performance aviation engines. Currently, ASTM (ASTM D 910) specifies three grades of avgas: Grades 80, 100 and 100LL (low lead). The ASTM has recently (2003) approved a new specification for a low octane unleaded avgas, Grade 82UL. Grades of avgas are identified by their nominal minimum lean mixture anti-knock rating. Historically, (until the 1970s) both the lean and rich mixture ratings were used, now only the lean mixture rating (octane number) is used. [FTR-3, p.41]. In practice, only 100LL is widely available. Grade 80 continues to be marketed but its distribution and availability is much more limited. Grade 100 is now rarely found. With continuing modernization of the aircraft fleet over time, the demand for Grade 80 continues to decline. It is expected that it will eventually reach a point when it is no longer economical to manufacture or use. Today avgas is used mainly by small aircraft and light helicopters; there is still a small but significant number of military and civilian transports that still use avgas.
The ASTM D 910 specification defines three grades of avgas, with names based on their antiknock quality as measured by Octane numbers. The grades are 80, 100, and 100LL, all of which contain tetraethyl lead (TEL).
Grades 100 and 100LL avgas are based on alkylate. As such, they are mainly synthetic products. Grades 100 and 100LL represent avgas identical in antiknock quality, but differing in maximum lead content. The lower lead content avgas, (100LL), is suitable for modern engines having a low tolerance to TEL. Few of the compounds in avgas come directly from crude oil. Toluene is often added to Grade 100LL to help meet the rich-mixture antiknock requirement. Because of the lower antiknock requirement of Grade 80 avgas, it may contain some straight-run gasoline distillate that has been subjected to additional purification after distillation. Light hydrocarbons, such as butane or isopentane are added to all grades to meet the minimum vapor pressure requirement. (Avgas must have vapor pressure of between 5.5 and 7.0 psi). Finally, additives are added: the required concentration of the appropriate tetraethyl lead (TEL)/ethylene dibromide (EDB)/dye mixture and any of the optional additives the manufacturer chooses to use [FTR-3, p. 65.1, Chevron]. Additives are hydrocarbon-based chemicals added to avgas in small amounts to maintain or enhance properties of performance or handling. Only additives approved by ASTM D 910 may be used in avgas. Approved additives are identified by their chemical formulas. For avgas Grades 80, 100 and 100LL, TEL, EDB and dye are mandatory.
Although the grade designations show only a single octane rating, antiknock quality is expressed by two values; the lean mixture motor rating and the rich mixture supercharge rating. The lean mixture rating is intended to simulate the lean air/fuel mixture of cruise conditions, and the rich rating simulates takeoff under rich mixture supercharge conditions.