I have kind of transitioned away from gasoline or Diesel and went to propane. A propane tank from your BBQ grill and a torch similar to one of these works well for me to start brush piles.
It seems you need to burn the trees as soon as they are cut (while the sap is still fluid) or wait about a year. I personally like to wait a year, then they burn really well.
That style of torch starts BBQ charcoal briquets really well and quick too! :-)