High Volume Low Pressure Engines

Overview
A LEGO HVLP engine is any LEGO engine that runs on high volume low pressure source of air flow and often use brick-made piston/cylinder assemblies. Examples of this airflow source include a vacuum cleaner (as in the case of the LEGO Vacuum Engine or LVE), leaf blower, industrial blower, etc. Most HVLP LEGO engines will run on either the suction or the pressure of this air, and some builders prefer pressure since a maximum theoretical pressure delta of 1 atm is achievable using suction because of the pressure of the earth's atmosphere whereas much higher pressures can be achieved using pressure as with a wet/dry vacuum cleaner with an exhaust hose attachment.

History
Because LEGO HVLP engines often utilize normal LEGO bricks for the piston and cylinder, the tolerance of the gaps between bricks is sloppy enough to allow inevitable leakage of air. It was therefore found that a source of air flow with a high volumetric flow rate neatly accommodated for this loss of pressure and flow.

See also: Overview of LEGO Engine History

Origin
The term "high volume low pressure" (HVLP) as used in reference to a LEGO engine was coined by Erin Lassley on his website in June, 1999 a few years after building his first working prototype engine. On his site he says the following:

"Today I managed to successfully complete a 20 minute trial run of this engine.  Compressed air was just way too much for this engine to handle.  It turns out that most of these LEGO engines run best with an HVLP setup (High Volume Low Pressure) namely, a wet/dry vac using the outlet side."