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4WD MODIFICATIONS - TECH TORQUE

ATKINSON AND MILLER CYCLE ENGINES
Older engine designs resurface to help petrol economy.

Electrification doesn’t mean the end of the internal combustion engine, but tomorrow’s passenger car engines are being designed primarily for hybrid-electric vehicles and as range extenders.

The days of the traditional, stand-alone
internal combustion (IC) SUV and 4WD engine are numbered, as more and more countries pass laws to outlaw the IC-only passenger vehicle.

In an effort to reduce the emissions of all engines, vehicle makers around the world have downsized their petrol and diesel engines. New small diesels, under two-litres displacement, have disappeared and have been replaced by turbocharged petrol engines and, increasingly, petrol hybrids.

Traditionally, 4WDs have been powered by engines shared with light commercial vehicles, but the increasing use of 4WDs as passenger car substitutes in metropolitan areas brings them into consideration for tighter emissions control.

Also, there’s pressure on light commercial vehicle makers clean up their emissions act, as well.

The net result may well be a return to petrol engines for 4WDs, albeit with hybrid powertrains. The North Americans have already headed down that route and the Chinese have stated that they won’t make any diesel 4WD wagons.

The 4WD market leader, Toyota, is currently expanding its successful and popular hybrid range and, in mid-2019, seems to have swerved away from hydrogen fuel cell powered future town vehicles to full battery electric vehicles (BEVs).

Toyota plans to introduce six new BEVs by 2025 and expects to have a solid-state battery with around twice the range of current lithium-ion batteries by 2022.

The company won’t comment on future product, but we understand that it’s currently campaigning its Australian rural dealers to explain hybrid-electric powertrains to Toyota’s bush customers.

Don’t be surprised top see a petrol-hybrid Prado, powered by either, or a choice of, Toyota’s new 2.5-litre four and 3.5-litre V6 petrol hybrid engines.

What’s common in these Toyota and other petrol hybrids is the use of an Atkinson-cycle engine, rather than a normal petrol engine.

 

The Atkinson-cycle engine

The original Atkinson-cycle engine of 1887 used a complex system of rods and journals to make the expansion (power) stroke longer than the compression stroke, achieving greater thermal efficiency than a normal four-stroke, Otto-cycle engine.

This Wikipedia animation shows the complexity perfectly.

The 2016 Infiniti variable-compression-ratio engine and other recent developments achieve similar thermal efficiency.

The downside of these designs is mechanical complexity and cost, so modern employment of the Atkinson principle typically uses variable valve timing to achieve economy.

Inlet valve opening is held during early piston travel in the compression stroke, ‘bleeding’ some compressed air out of the cylinder and reducing ‘pumping’ losses in the process.

The downside of valve-action Atkinson engines is a reduction in power output, compared with a normal four-stroke, but with great economy benefits.

Also, Atkinson-cycle engines lack to responsiveness of normal engines.

The drop in output and response makes an Atkinson engine impractical for conventional IC-engined vehicles, but for a hybrid that has an electric motor as well as an IC engine the trade-off isn’t as important as the economy benefits.

The electric motor supplements lift-off and acceleration power and torque requirements, so performance doesn’t suffer.

Additionally, variable valve timing allows the Atkinson-cycle function to be turned off or scaled back, as power demands may dictate.

Out testing at OTA of 4WD hybrids shows that their economy is better than that of diesels, with similar or better performance.

 

The Miller-cycle engine

An Atkinson-cycle engine differs from a Miller-cycle engine in that the Miller-cycle engine uses some device to push air into the system – a supercharger or turbocharger – while the Atkinson-cycle engine is naturally aspirated.

US engineer Ralph Miller patented his Miller-cycle engine in the 1940s, but it didn’t appear in automotive engines until the late 1990s. Mazda and Subaru employed Miller-cycle engines briefly, as did Caterpillar with its ACERT truck engine range.

Like an Atkinson-cycle engine that uses variable valve timing to delay inlet valve closure a Miller-cycle engine leaves the intake valve open during the first part of the compression stroke, but the piston is compressing against the pressure of a supercharger or turbocharger, so there’s less compressed air loss than there is in an Atkinson-cycle engine.

Because the combustion chamber is over-fed with charge air, pushing some of the charge air back out into the intake manifold is entirely planned.

The effect can be increased efficiency, at a level of about 15 percent, because the piston gets the same resulting compression as it would in a standard engine, but for less work. However, that’s the case only when turbocharging can compress the charge using less energy than the pistons would use to do the same work.

As with an Atkinson-cycle engine, variable inlet valve timing allows the Miller-cycle function to be tuned or even switched off to suit engine revs and load.

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