Tech Tip: Vacuum readings on hybrid vehicles

Do you use a vacuum gauge and/or a vacuum transducer and scope as part of drivability diagnosis?  If so, there are some differences in hybrid vehicle engines that you need to be aware of.

Hybrid vehicles typically are stated to use an “Atkinson Cycle” engine.  This isn’t really the case, but they do use a vehicle that behaves like an Atkinson Cycle engine.  A true Atkinson Cycle engine has a shorter intake/compression stroke length in comparison to the power/exhaust stroke length.  The idea is it allows the engine to capture more of the energy from the burning air/fuel mixture than a traditional Otto cycle engine does.  The reality, however, is hybrid vehicles aren’t using true Atkinson Cycle engines. 

So what are hybrid vehicle using?  The typical hybrid being advertised as having an Atkinson Cycle engine is really utilizing valve timing to simulate the Atkinson Cycle.  By holding the intake valve(s) open into the compression stroke it allows a portion of the air/fuel mixture to be pushed back into the intake manifold.  The intake stroke and compression stroke are still the same length as the power and exhaust stroke, however with the late intake valve closing it makes the effective length of the compression stroke shorter.  This decreases the compression losses within the engine and also allows more energy from the power stroke to be captured.  The down side to this is that it decreases the torque output of the engine.

When vacuum testing an engine that operates in this manner you need to be aware of these differences.  Every time a cylinder begins the compression stroke some of the air/fuel mixture in that cylinder will be forced passed the open intake valve and into the intake.  Normally this pressure being pushed back into the intake would be a sign of a burnt intake valve, but on these engines it is completely normal.  To complicate things more, these vehicles typically include variable valve timing which means the amount of late intake valve closing can change based on engine operating conditions. 

Be sure to keep this in mind if you utilize intake manifold vacuum testing as part of your diagnostic routine.   

Information provided by: AR&D