2.0 liter DI-Atkinson

[dwightlooi]

The 2.0 Liter LNF is a nice little engine in turbocharged form. But as a base engine on a small car like the upcoming Cruze it may not exactly be the right prescription.

However, the 2.0 liter block can form the basis of a nice little engine designed for economy instead of performance. And one of the ways we can optimize it for gas sipping duty is to incorporate Part-Time Atkinson Cycle operation using cam switching hardware already tested on the HCCI concept 2.2 engines. Since the engine block and most components are shared with the LNF it also reduces costs and inventory if the vehicle will also have a turbocharged high performance stable mate.

Atkinson Cycle (and its supercharged Miller Cycle variant for that matter) operates on a principal that an asymmetric intake and exhaust stroke extracts the best thermal efficiency out of a piston engine. The physical piston stroke is no different from a regular engine. However, the intake valves on an Atkinson Cycle powerplant does not close until the piston is well into its compression stroke. This causes part of the intake charge to be kicked back out of the cylinder during the compression stroke and there is also no effective compression for the portion of the compression stroke where the intake valves remain open. For instance, if you have a 86 mm physical stroke, but keeps the intake valves open for 30mm of that stroke, you have in essence created an engine with a 56mm compression stroke and effective dispacement, but a 86 mm power stroke. This helps extract more energy by simply allowing the pressurized gases more time and distance to do their work before venting them.

The downside is that the "effective displacement" of the engine and hence its power output is similar to that of an engine roughly 2/3rds the physical size. The plus side is that the efficiency is often times better than an engine 2/3rds the size. Example: The Prius's 70hp 1.5 liter engine is the Atkinson Cycle version of the 108 hp 1.5 liter 1NZ-FE engine in the Yaris/Vitz/Echo.

However, all the aforementioned downsides of an Atkinson Cycle engine assumes that the engine is always operating in Atkinson cycle mode. With cam switching technology (ala VTEC, VVTL-i, MIVEC and whatever GM's implementation ends up being called) we can switch between two camshaft lobes. We can operate in Atkinson cycle or a conventional Otto Cycle at will by changing between a cam lobe which closes the intake valves very late and one which doesn't. This will create an engine which can switch to a cruising or light load mode with better economy than a typical 1.4 iter engine, and one which restores the performance potential of a 2.0 liter direct injection powerplant. The output will probably by around 160~170 hp in Normal Mode and about 100 hp in Atkinson Mode. Not bad really, for a small car.

BTW, the Civic's 1.8 liter 140hp four is already pulling a similar trick.

Edited August 23, 2008 by dwightlooi