Biogas engines need to be activated by a redstone current like any other BuildCraft engine.
However, biogas engines have one additional quirk: They cannot explode from overheating, but they need a minimum operating temperature. You can fill a biogas engine up to the brim with delicious fuel and it will still refuse to start up. The reason is quite simple that it needs to reach a certain temperature before it can begin to burn fuel. To start the engine when it's new or whenever it has cooled down - for example when it has used up all fuel and has idled for too long - you will need to dump lava into the secondary tank on the right hand side. The engine will then start to heat up and once it has reached minimum operating temperature it will start to produce energy. Producing energy generates itself heat, so after the initial warmup a running biogas engine will keep itself heated without using up any further lava from its supply. Also, if the activating redstone signal is interrupted, the burning of fuel will pause completely: fuel will not be lost, but the engine will eventually cool down and require the consumption of lava to start up again. As long as the redstone signal is activated the engine will not cool down, even if it has no fuel left.
However there is one important exception: If you use just plain water to power your engine, the engine will not generate enough heat to counter the natural heat dissipation. A biogas engine that uses water will burn quite literally through its whole lava supply, while one constantly supplied with liquid biomass or seed oil will happily chug along all day without so much as touching the lava. A water fueled engine will invariably use up all the lava in the secondary tank and stop producing energy. However, you can pipe in lava using BuildCraft pipes if you choose to do so.
Biogas engine can be powered by Water, Milk, [Bug in current version, Can't use Milk] Honey, Apple Juice, Seed Oil, or Biomass. The fuel and lava in the biogas engines internal storage can be collected with a Pipette.Biogas engines can be preheated with other fuels.
(After version 2.0 of forestry milk will require constant lava usage)