Can a small, turbocharged flex-fuel spark-ignited engine perform as well as a heavy-duty diesel engine -- but at a much lower cost? Ricardo Inc. and its partners are getting ready to demonstrate exactly that.
Ricardo calls the technology Ethanol Boost Direct Injection and it's said to be scalable from small passenger cars up to large commercial trucks.
As we explained last year when we first looked at Ricardo's EBDI technology, it’s taken spark-ignition technology decades to catch up to diesel efficiency. Diesel is so efficient and powerful because it contains more energy per gallon of fuel than gasoline, and it combusts using compression ignition, which is the tremendous frictional heat generated from the extreme compression of air in the cylinder. Such high temperatures and pressures produce the large amounts of torque that trailer-towers love, but they inhibit thorough mixing of the fuel-air charge in the cylinder, which leads to incomplete fuel burns that produce soot and other pollutants. Conventional gas and flex-fuel engines use spark ignition to detonate the fuel-air mix when the charge is optimally distributed throughout the cylinder chamber. That has two results: The mix burns cleaner, and it burns with less relative power than diesel.
Ethanol, however, has a higher octane and heat-of-vaporization point than gasoline, meaning it combusts at a higher temperature and with greater force (and compression) than gasoline, while also having a greater capacity to cool the fuel-air mix in the cylinder before combustion. This allows a larger charge to be drawn into the cylinder before ignition. This inherent efficiency is what enables a smaller-displacement engine to perform with the same power as a bigger motor -- if the engine is built to take advantage of it.
Ricardo’s EBDI engine changes its combustion cycle to match the fuel blend and it's built to withstand high compression ratios. This, together with the use of direct injection, exhaust gas recirculation and turbochargers, makes it almost as efficient as a diesel engine, Ricardo says.
For now, Ricardo starts with a 264-horsepower, 222-pounds-feet of torque 3.0-liter direct injection gasoline V-6 engine (from a Chevy Equinox crossover) that's been enlarged to 3.2-liters, by increasing the stroke length. Using that engine, Ricardo will replace the 6.0-liter V-8 gasoline and 6.6-liter Duramax V-8 diesel in two GM Heavy Duty pickup trucks starting in May.
According to Luke Cruff, a Ricardo chief engineer involved in the program, power output will vary depending on the fuel blend used. Running on gasoline only, the EBDI engine is said to produce 400 horsepower and 570 pounds-feet of torque. Using E85 (15% gasoline, 85% ethanol), that figure jumps to 450 horsepower and 660 pounds-feet of torque.
Front view of the EBDI 3.2-liter V-6
Peak torque is available down low, like a diesel, at 1,500 rpm to 3,000 rpm, depending on the fuel mix. The engine's RPM range is diesel-like too, topping out at 5,000 rpm.
"An [EBDI] engine like this doesn't have to run [at] high [rpms]. It can be downspeeded because the torque is so high," Cruff said.
To realize the best performance balance between mileage and power, the optimal fuel blend to run is somewhere in the middle, using an E40 to E50 mix. That's the point where the engine returns gas-like fuel economy. That's because ethanol has about a 33% lower power density compared to an equivalent amount of gas, meaning it takes roughly a third more ethanol to drive a certain distance than you'd need if you used only gas. This is the reason that today's flex-fuel cars and truck have lower EPA mileage ratings than for gas-only versions.
Downsizing and turbocharging helps reduce some of the lost fuel economy that comes with Ethanol. The smaller engine has lower pumping losses -- the power needed to move air in and out of the cylinders -- and boosting allows the engine to run at a higher load.
Rear view of the EBDI 3.2-liter V-6
"If you put in an E40 blend, it's like pushing gasoline up to 97 octane," Cruff said. "Adding ethanol is like adding an octane booster in the tank. You still get most of the performance benefits that you'd get on E85 but without as large of a fuel consumption hit."
Since ethanol blends like E40 aren't common, Ricardo has teamed up with Growth Energy, a trade group that hopes to see variable-blend ethanol pumps installed at gas stations so that drivers can choose the best blend of fuel for their needs.
"If I'm towing, I'll use a higher ethanol blend for more power," Cruff said. "But if I'm cruising down the highway, then I'll put in lower-octane gas for better fuel economy."
The EBDI engine also varies the amount of boost, cam timing, spark timing and fuel injection timing to optimize performance for various blends of ethanol and gasoline. It operates at about an 11:1 compression ratio versus around 17:1 for a diesel.
A future version of the engine will receive adaptable variable nozzle turbos that can optimize boost for low and high-speed operation, to further improve fuel economy and power availability.
This picture compares the standard direct injection 3.0-liter V-6 piston and connecting rod (top) to that used in the EBDI 3.2-liter V-6 (bottom). Note the sturdier construction of the 3.2-liter hardware -- particularly the connecting rod -- to handle the higher compression ratio and torque of the EBDI engine.
Like a diesel engine, the EBDI engine uses exhaust gas recirculation to control cylinder temperatures. Instead of limiting the formation of nitrogen oxide -- which needs to be scrubbed from diesel exhaust to meet tough new federal emissions regulations -- the EGR is one more way to help prevent damaging engine knock. This use of EGR means that purchase costs are much lower because it doesn't require an expensive diesel exhaust aftertreatment system. It also adds a charge air cooler to further manage air temperatures before it enters the engine's intake manifold and is combined with fuel in the cylinder.
The EBDI engine uses a simpler fuel-injection system that runs at only about 10% of the pressure required for a diesel fuel injection system. That's because diesels require fuel injection pressures of around 30,000 psi to finely atomize the fuel before it combusts -- again, to meet emissions. The fine timing and spark ignition setup of the EBDI engine allows for the lower pressures.
Not using EGR (and urea selective catalytic reduction) for emissions and the simpler fuel injection system means the EBDI engine should cost half as much as a comparable diesel but about $4,500 more than a conventional gas engine.
"We're trying to make a point by choosing to demonstrate this engine in a heavy duty truck that this could help improve the fuel economy of smaller trucks too," Cruff said. "Fuel economy numbers are going up. If a company wants to keep selling trucks, they'll have to sell lots of small cars or they're going to need technology like this to get there."