Remember that diesels already operate at very high cylinder pressures. You can't just keep increasing the pressure without disastrous results. But you can make the best use of the volume that gets compressed. Scavenging means that instead of having 80 or 90% air in your compression stroke, you can get close to 100%. More air, more oxygen. More oxygen, more fuel. More fuel, more power. I know it sounds counterintuitive, but that's what my power mechanics teacher said.