Why does it take so much fuel to start a jet engine?

I just watched an Abrams tank documentary and it said something like it takes "1 liter of fuel for every mile but 39 liters for a startup." Is it true of turbine engines in general that they need a lot of fuel to start? If so, why?

• I heard the same about our tanks, though those had piston engines. I always felt those numbers are heavily exaggerated, because 1: there would be hardly any place left for air in the motor an 2: this amount if fuel needs 350m^3 of air to burn completely. Or: given the excess of air on a diesel/jet engine, that would create a storm around the tank. I'd first check if the number is true. I can imagine this is the amount a dry tank sucks in to flush all its fuel lines. Nov 16, 2017 at 5:12
• A common urban myth is that X takes so much energy to start up, you should just leave it running. Fluorescent lights burn 10 minutes of power to start, computers use 30 minutes of energy to boot up, it takes more energy to rechill your house than to run the A/C all day, yada yada. They are all not only false, but thermodynamically impossible. Nov 17, 2017 at 4:14
• @Harper for some devices there is some truth in them, a combustion engine need to get up to speed to be able to idle, an electric motor will suck the most current when stalled/stopped, a fluorescent needs to heat up the electrodes and charge an inductor before it can try and initiate a spark to light it, a computer will exercise all its components on startup, etc. Granted the extra energy needed is on the order of seconds (if that) rather than minutes/hours. Nov 17, 2017 at 9:22

The first figure, 1 litre per mile, is very wrong. The second, 39 to start, is most probably the fuel required for a warm up from a cold start. The Abrams tank weighs 62 tonnes, so you won't get 1 mile on a litre. Its fuel consumption is quoted as 0.6 mpg, or 1.66 gallons per mile, which is 6.3 litres per mile. Quite a difference.

The engine is the Honeywell (previously Lycoming) AGT 1500. It has a two spool gas generator, with a two stage power turbine. It features a heat exchanger through which the HPC exit air flows, to preheat it from the exhaust gas, prior to combustion, to improve engine efficiency.

The engine has a tactical idle that keeps it running at 1500 rpm instead of 950 so that it can rapidly accelerate from a stationary position if needed to avoid or engage a hostile force. In tactical idle it consumes 30+ gallons per hour, instead of 10 at the non tactical idle setting.

It's stated that "in 1990, Project on Government Oversight in a report criticized the M1's high costs and low fuel efficiency in comparison with other tanks of similar power and effectiveness such as the Leopard 2. The report was based on data from U.S. Army sources and the Congressional record."

Some critics say the tank fuel consumption is so poor, that in combat, it has to often wait for the fuel supply logistics to catch up. Other reports deny this. (See paragraph at the very top of the page).

The M1 Abrams was chosen after a competition in June 1973, between the Chrysler Corporation and the Detroit Diesel Allison Division of the General Motors Corporation to build prototypes of a new tank designated M1, and later named the Abrams tank. In November 1976, it was announced Chrysler won the competition.

The M1 was faster and more maneuverable than its predecessors in the M60 series, while offering a lower, smaller silhouette. Besides the substantial gains in performance, the Textron Lycoming AGT-1500 turbine engine was far more reliable than the diesel tank engines then in use with the Army. There was still another benefit. The engine change, despite a penalty in fuel consumption, also resulted in much quieter operation, so much so that soldiers encountering the tank in early maneuvers started calling it "Whispering Death".

The Abrams wikipedia page also states a figure of 6.3 liters per mile, and also states the value of 38 liters to start. As suggested by user3528438 (thank you), the 39 litres (10 US gallons) is quite probably the fuel required to complete a warm up from a cold start. At the tactical idle setting, this would be the fuel consumed in about 15 to 20 minutes.

• If it's a cold start it might take minutes to warm up. If a tactical idle is 30 gallons per hour, 39 liters for a warm up run isn't too bad. Nov 16, 2017 at 15:00
• Okay. A now about the jet engine... Nov 16, 2017 at 15:21
• @user3528438. Yes, thinking about it since writing the answer, I think that's what the 39 litres is required for - not starting, but the warm up. I will modify the answer to include your comment. Nov 16, 2017 at 19:49
• @Peter. The engine being discussed is a gas turbine, the Lycoming (now Honeywell) AGT 1500. I will modify the answer to make that clear. Nov 16, 2017 at 19:51
• Outrunning the supply chain is a compliment, not a criticism. This has been a problem since WWII and reflects the good performance of the tanks. And it's more a case of being able to secure the supply chain. The precious tankers could follow close behind the tanks, but would be cut to pieces by uncleared enemy units who hid from the tanks, or even local irregulars, farmers with guns etc. Nov 17, 2017 at 4:06