Turbo-free engine boosting

Between regulatory mandates and consumer demand for efficiency, motor vehicle producers need to get every bit of power possible out of every drop of fuel that cars and trucks burn. For most major automakers, that means shrinking engines – replacing V-8s with V-6s and V-6s with I-4s – then adding turbochargers to make up for the lost power.

Toyota Motor Corp. is taking a different approach. The company is using a new line of engines to boost efficiency by as much as 10%. Its next-generation 1.3L and 1L small engines increase compression to previously unapproachable levels, introduce technologies to fight off of the negative effects of that increased pressure, and use a 132-year-old engine technology in its non-hybrids for the first time.

“By combining the thermal efficiency improvements we’ve made in our hybrid engines with the technologies we’ve developed for our conventional internal combustion engines, we are now able to achieve the thermal efficiency of hybrid vehicles in our internal combustion engines in non-hybrid powertrains,” says Shoji Adachi, project general manager at Toyota Powertrain Development and Production in Toyota City, Japan.

Adachi says the automaker has fine tuned the combustion process to expand engines’ peak efficiency ranges. Instead of using boosting strategies to keep engine speeds low, the company is effectively making sure that engines can be efficient at low, medium, and high rpms.

“They are taking a completely different path than Ford, GM, BMW, and all of these people who are working on downsizing and turbocharging,” says Zoran Filipi, the Timken endowed chair in vehicle system design at Clemson University in Clemson, S. C. “They’re looking at improving every step of the combustion process without adding new systems.”
 

High pressure, no knock

Boosting engine compression ratios to increase power is nothing new. Ask hot-rodders, and they’ll share stories about a customized Chevy short block from 1965 or a Ford Cleveland 351 from the 1970s. By swapping out cylinder heads and making other engine modifications to boost compression, customizers could get another 50hp out of their engines, giving them an edge in street races. Adding more fuel and air to the cylinder creates a stronger explosion when the spark plug ignites the mix, creating more usable power from the engine.

However, if engineers increase the compression too much, the engine can’t maintain a smooth compression-combustion cycle. Fuel ignition happens too early or too late, creating knock. Over time, this can damage engines, so designers tend to increase compression ratios until they hear knock, then dial back a bit.

“Anything you can do to fight knock, to limit knock, helps support a higher compression ratio,” Filipi says.

Instead of rolling compression down when they heard knock, he says Toyota engineers studied the phenomenon to finds ways of fighting it, allowing for even higher compression.

The first technique was to pump some exhaust gas back into the cylinders. By inserting cooled exhaust gases back into the engine, Toyota slows down the combustion process and prevents knocking. Cooled exhaust gas recirculation (EGR) is a common technology for lowering emissions.

Adachi says Toyota is expanding the amount of EGR used in the engines to better control in-cylinder combustion.

Slowing combustion times with cooled EGR can create two new problems – less engine power and an increased possibility of knock. On one hand, the inert gas lowers the potential, but on the other, increasing the amount of time it takes to burn fuel increases the chances that a knock can occur, Filipi explains.

“Knock is a result of chemical kinetics,” Filipi states. “Molecules start to break up and form radicals, and at some point you have auto-ignition. It’s a chemical process that has a certain time scale. So if you shorten the time, if you don’t give it enough time, the reaction doesn’t happen.”
 

Speeding up combustion

To counter the negative effects of cooled EGR, Toyota had to find ways to speed up the combustion process. The faster the combustion, the less the chance for knock, and compression ratios can be higher.

Adachi says the easiest way to speed combustion is to increase air movement within the engine’s cylinders. As anyone who has fanned a campfire can tell you, flames propagate faster when air moves around an enclosed area. By controlling the shape and rate of the airflow, engine designers can control the speed of combustion.

“As much fuel as possible will be burned as quickly as possible to prevent knocking before it can occur,” Adachi notes.

Again, there has traditionally been a tradeoff – the more air turbulence in the cylinder, the less air entering the cylinder to mix with fuel and ignite. Fuel-to-air ratios are precise, so less air equals less fuel, leading to less power from the engine.

“We managed to achieve high tumble flow strength and high air flow,” Adachi says. Designs improvements to intake and outtake valves allow for high-speed, very turbulent air flow in the cylinders.

In addition to faster combustion and cooled EGR, Toyota employed a third technique to limit knock. Knock tends to happen at the top of the cylinder because constant combustion there makes that portion of the engine hotter than the middle and bottom sections.

So, using a technology it calls precision cooling, Toyota changed materials and insulation in the cylinders to lower temperatures at the top.

“It’s very straightforward. You wonder why nobody has done it before,” Filipi says. “They cool the middle section less generously, keeping the temperature of the middle of the cylinder close to the top.”
 

Atkinson cycle combustion

In addition to the higher compression, faster combustion technologies, Toyota plans to expand its use of the Atkinson cycle in the small engines. Used exclusively on hybrids today, Atkinson changes how engines pull fuel into the cylinder, compress that fuel, and ignite the fuel-air mix.

Invented in England in 1882 by engineer James Atkinson, the engine system uses an imbalanced approach to the basic steps of engine operation. Traditional Otto-cycle engines have four even strokes for fuel-air intake, compression, combustion, and exhaust. Atkinson engines have longer combustion cycles, allowing for a more complete burn of fuel at lower power levels.

With Atkinson, the engine’s valve doesn’t close immediately following intake. As the piston rises during compression, some of the air-fuel mixture goes back into the fuel system, so less fuel remains in the piston during ignition. Following compression, when the spark plug ignites the fuel, Atkinson engines allow the piston to move further down the cylinder, taking full advantage of the combustion cycle.

Because it uses less fuel, Atkinson produces less power, especially at lower speeds. In hybrids, that isn’t a problem because battery powered electric motors handle much of the low-speed workload.

Adachi says the new Toyota engines will be able to operate in either Atkinson or Otto cycles. When the vehicle needs more power, the engine’s variable valve timing (VVT) system will operate as a traditional Otto engine. At cruise, when the power requirements are lower, the timing can change, allowing for Atkinson cycle.

“With conventional engines, sufficient performance will not be achieved” using only Atkinson, Adachi says. “If you want power, you need to use variable valve technologies to return to a standard combustion cycle. For this to happen, expanded VVT operating angles and electronic VVT are necessary.”

Adachi says the new small engines will go into use in Japan later this year and will appear in global vehicles soon after.

Toyota Motor Corp.
www.toyota.com

About the author: Robert Schoenberger is the editor of TMV and can be reached at 330.523.5381 or rschoenberger@gie.net.