High-efficiency gasoline engines

Dr. Neil Canter, Contributing Editor | TLT Tech Beat October 2010

New technology is helping to improve fuel economy and lower emissions.

KEY CONCEPTS
• A program known as HEDGE uses cooled EGR to suppress engine knock, improve fuel economy between 5% and 30% and reduce emissions in gasoline engines.
• Cooled EGR also eliminates a phenomenon known as LSPI that can occur randomly and leads to premature engine failure, particularly with turbocharged engines.
• A new consortium to evaluate the impact of the engine oil and fuel on LSPI is being formed and will kick off on Jan. 1, 2011.

The emphasis on increasing fuel economy and reducing emissions is placing greater demands on engine manufacturers. As a result, engine sizes have been reduced to improve fuel economy, and measures have been taken to protect the automobile’s emission system.

Last month, TLT featured a special report on the development of the most recent ILSAC gasoline engine oil specification, ILSAC GF-5 (1). Among the issues is a tradeoff between using friction modifiers to improve fuel economy and being able to pass engine deposit tests. Two key tests that are challenging to pass are the Sequence VID fuel economy test and the TEOST 33C. The latter is used to evaluate the effectiveness of the engine to minimize turbocharger deposits.

Light-duty diesel engines have traditionally exhibited superior fuel economy performance compared to gasoline engines. Dr. Terry Alger, manager of the Advanced Combustion and Emissions section in Southwest Research Institute’s (SwRI) Engine, Emissions and Vehicle Research Division in San Antonio, Texas, says, “In 2001, SwRI’s Clean Diesel consortium was looking at reducing emissions from heavy-duty diesel engines. As part of this process, we examined the use of gasoline technology with a 3-way catalyst as a low-cost option. We found that cooled exhaust gas recirculation (EGR) could suppress knock in gasoline engines, allowing them to meet heavy-duty performance and efficiency goals.”

Attention turned to how the performance of a gasoline engine can be improved to achieve improved fuel economy and lower emissions. Alger explains, “We believe there is an opportunity to utilize a comparable approach to enable a gasoline engine to be as efficient as a diesel engine.” 

A technology is in the process of being implemented to meet this goal.

COOLED EGR
SwRI initiated a program known as High-Efficiency, Dilute Gasoline Engine (HEDGE) to develop a more effective gasoline engine. The objective involves utilizing cooled EGR to achieve the goals noted in Figure 3.


Figure 3. Cooled EGR improves the efficiency of gasoline engines by eliminating LSPI, reducing emissions and reducing exhaust temperatures. (Courtesy of Southwest Research Institute)

Alger says, “We found that using EGR at a rate between 25% and 50% suppresses engine knock, improves fuel economy between 5% and 30% and reduces emissions. A heat exchanger is used to drop the temperature of the EGR stream from 700 C to between 90 C and 150 C.” 

This result can be achieved with a higher compression ratio between 11:1 and 15:1, which enables the engine to obtain more mechanical energy from the air, fuel mixture and, therefore, be more efficient.

Another benefit of using cooled EGR is the elimination of a phenomenon known as low-speed pre-ignition (LSPI). Alger says, “LSPI is a random and sporadic pre-ignition event that can occur in a cylinder for a few cycles and then go away only to return at a later time. For example, LSPI events occur in an alternating fashion with a pre-ignition cycle followed by a normal cycle for up to 15 or 20 engine cycles and then disappear only to return as many as 30,000 cycles later. A typical frequency for LSPI is between 4 and 20 events per 30,000 engine cycles, depending on the engine speed and torque.”

Alger indicates that the origin of LSPI is not known, but it has become a major problem since turbochargers have been used to boost the performance of gasoline engines. He adds, “LSPI causes heavy engine knock and can increase the peak pressures in the cylinder by two to three times. This effect can lead to holes in the piston and break other components, which can cause complete engine failure in one to two cycles.”

While the HEDGE consortium was established eight years ago to investigate the effect of cooled EGR on engine efficiency, the attention turned to LSPI over a year ago. Alger says, “In engines showing 4-5 LSPI occurrences per 30,000 cycles in the absence of EGR, the use of 5% EGR reduced the number of LSPI episodes to 1 per 30,000 cycles. If EGR is raised to 10%, then LSPI is eliminated at that load. At higher loads, up to 15% cooled EGR is required.”

Alger suspects that fuel and the engine lubricant also contribute to LSPI. A study was done comparing 87 octane gasoline to blends of this fuel with 10% ethanol, a high level of aromatics and a low level of aromatics.

Alger says, “The highest frequency of LSPI was observed with 87 octane gasoline. Introduction of ethanol reduced the occurrence of LSPI by 50%, while the presence of 87 octane with a high level of aromatic content also generated some LSPI. No LSPI was seen with 87 octane gasoline prepared with a low level of aromatic content.”

Alger indicates that engine oil is a big contributor to knock as it displays a cetane number similar to diesel fuel. He indicated that some engine manufacturers and other researchers believe that the oil is also causing LSPI.

Alger says, “We are launching a new consortium to evaluate the effect that engine oil and fuel composition have on triggering LSPI. The goal is to come up with a test that engine manufacturers can use to evaluate the impact of the lubricant or fuel in contributing to LSPI.”

Alger believes that the engine manufacturers then will include such a test in their lubricant specification. The new consortium kicks off on Jan. 1, 2011.

Further information on HEDGE can be found at http://www.hedge.swri.org or by contacting Alger at talger@swri.org. 

REFERENCE
1. Canter, N. (2010), “Proper Additive Balance Needed to Meet GF-5,” TLT, 66 (9), pp. 10-18.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.