Its more a matter of scale than anything else. In its classic form, a Molotov is a glass container containing, roughly, a liter of gas/oil mix. On its own, its not enough to kill the engine or crew. Like I mentioned earlier, modern tank designs do take into account the use of a Molotov and run-off points are provided. Tank crews are also trained to exit the area and get away from the fire.
Portable flamethrowers use a mix that tends to stick to things. But again, the design helps protect from the fire.
Best choice would be an attack aircraft that dumps a load of naplam on the tank. Drop enough and you will get the effects you are looking for, but in today's military, its a wasteful use of a combat aircraft to kill a single tank by that means. A Maverick missile, a Rockeye cluster bomb or even Copperhead or 155mm HE would be a more efficient choice.
The air intake on the Abrams is protected by two things, the large size of the turret helps shield the air intake from a direct hit and the intake has three pieces of armor that are angled slightly in and 'lipped" to drain towards a run-off point. Tests were run at Fort Knox and Aberdeen Proving Grounds during the developmental workup and the flame weapons used had little effect.
The primary purpose of any flame weapon is more mental than it is physical. Flame scares the bejesus out of any one, especially when you see someone deliberately trying to burn you. It is this very instinctive fear that has lead to the near absence of flame throwers on the modern battlefield. The reaction of the enemy to pour as much fire as possible at the operator. A Marine Corps study conducted after WWII confirmed that the most hazardous position in the Corps was a flamethrower operator in a island assault, his expected life span was measured in minutes.
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The reason that the American Army does so well in wartime, is that war is chaos, and the American Army practices chaos on a daily basis.
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