14 to 1 compression ratio on new Camry Hybrid

@Tideland Prius is usually good at digging up things like this. @Elektroingenieur may have insights too.

 

The engine has a hybrid of port and direct injection. The DI allows higher compression ratios than the older port injection did. IIRC, Mazda's SkyActiv is 13:1 in the US and 14:1 elsewhere.
The variable valve timing will allow the Camry hybrid engine to run a lower compression ratio for an Atkinsonized cycle for efficiency.

 

Atkinsonization is a product of valve timing. Fuel injection type doesn't matter. Direct injection gas engines in consumer cars is newer technology than hybrids and Atkinsonized engines. The first Prius used port injection, and the latest one still does. In fact, what Atkinsonized Otto cycle engines do was done in the Miller cycle back in 1954.

Fuel vapor getting pushed back isn't an issue. It just gets sucked back into the cylinder next time around, and the air flow from the intake is still going just one way. A carbon filter captures any lingering after the engine is shut off.

Mazda's SkyActiv engines are Otto cycle, and are 13:1 compression on US regular. They are 14:1 in other markets. The 4 cylinder in the new Camry is 13:1. So the hybrid ICE is likely doing some degree of Atkinsonization all the time.

Port injected engines require lower ratios with 87 AKI because the fuel is getting compressed with the air, which heats it up. When the air:fuel mixture is ignited by the spark, there is further compression by the burning fuel of the unburnt mixture. That extra compression can be enough to trigger ignition of the unburnt fuel, so there is now multiple ignition points within the cylinder. This is knocking.

Since direct injection doesn't add fuel to the cylinder until the air is already compressed, the fuel itself hasn't been heated up. Squirting it into the cylinder actually will cool the air some.

Well, a true Atkinson Cycle engine doesn't adjust any valve timing. Instead the length of the piston strokes is physically changed. The pistons aren't directly connected to the crankshaft. There is a cam in between the two that lets the intake/compression strokes be shorter than the expansion/exhaust ones.

Since Miller already figured out how to get an effective short compression in the '50s for lower cost, a true Atkinson engine isn't used in any cars. Honda does use it in a stationary generator though. It adds mechanical complexity, but has lower internal friction. The angle of deflection for the piston rod is smaller than in typical piston engines. This means less sideways force of the piston head against the cylinder wall.

 

The compression ratio is calculated by measuring the volume of the cylinder volume at BDC and then dividing this volume by the cylinder volume measured at TDC.

http://www.andersonautomotiveenterprises.com/PDFs/techtips/CompRatio.pdf

 

it is however, the reason for a long intake tract on the prius, so that fuel/air mixture does not get released to the outside air. Owners who try to put 'shorty' intakes tend to get codes due to odd F/A mixtures.

 

These are Otto cycle engines which mimic the differing expansion and compression ratios of the Atkinson cycle through valve timing. In truth, they are actually Miller cycle engines without a supercharger. An Atkinson cycle engine has those aforementioned cams between the pistons and crankshaft. Atkinson developed this engine to get around Otto's patents. It trades power for efficiency, and that is so for Atkinsonized Ottos too.

Variable valve timing is an old technology that has been steadily improving. It is reaching the point in which an Otto engine can run at a full Atkinsonized cyslce to improve efficiency under low loads

I agree that the 14:1 is an expansion ratio, but the variable valve timing could mean it that a 13:1 compression ratio is possible for high power demands.

In some countries, the regular is higher octane than what we get in the US. All else being equal, the higher compression will use less gas in addition to making more power.

 

I glanced through Toyota’s New Car Features book for the 2018 Camry HV and the section on the A25A-FKS engine in their course TIN517B, Fall 2017 New Technology Update, but I didn’t see anything; it’s not relevant information for servicing, since compression tests are done in a special diagnostic mode.

Perhaps Toyota will submit an SAE technical paper or publish an article in the Toyota Technical Review.

 

It is too new for those details to be out yet.
The ICE Camry 4 cylinder likely does run at its 13:1 under high load. Being based upon that engine, the hybrid could run as high, in theory. The effective highest compression ratio it does reach is likely lower than that, since the car has an electric motor helping the ICE out, but only Toyota knows what that actual value is.