Unintended Consequences Drive GDI Engines to Your Shops: Knock, for Customers & Service Writers

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Here’s an exchange between a customer and a service writer:

Customer, “My fairly new vehicle seems to have lost power and fuel mileage. Can you help?”

Service writer, “Our check indicates that your knock sensor(s) caused changes to save your engine from damage.”

Customer, “But I haven’t heard knocking! What’s going on here?”

Both are likely correct. Read on.

For the Service Writer — Knock Evidence

Knock can destroy turbocharged gasoline direct injection (TGDI) engines — a dominant design including all Ford EcoBoost engines.

Research showed that, “[With TGDI] downsizing and increasing power output, the failure mechanisms started to become more clear … The rods tend to fail first, and the second ringland follows … people that lose their motors on the highway … speak about a cloud of black smoke … caused by a LSPI [low speed pre-ignition or super-knock] event.”

Digitaltrends.com/cars explained the knock difference between gasoline and diesel engines, “In a gas engine, the air and fuel is compressed and at a critical point in the timing of the cycle, a spark plug ignites the gas.

“But in a diesel engine, there are no spark plugs. When diesel fuel and air are squeezed enough, the extreme compression generates enough heat that the mixture essentially spontaneously combusts. This is known as ‘compression ignition,’ and it’s the basis of how a diesel engine works.

“When a gasoline engine has combustion ignition, we often call it ‘knock,’ and it can ruin the engine.

“But diesel engines are built to take advantage of it.”

In GDI engines, more gasoline combined with higher compression produces unburned fuel droplets that cause increased combustion chamber hot-spot deposits. These increase knock — as verified by more than 2,000 Society of Automotive Engineers technical papers and journal articles that contain all the words “gasoline direct injection knock.”

Super-Knock Can Catastrophically Destroy Engines

If knock can damage an engine, then super-knock, or LSPI, can cause a catastrophe.

A new 2017 SAE Paper No. 2017-01-0686, named “Effect of Timing and Location of Hotspot on Super Knock during Pre-ignition,” reported, “Pre-ignition in SI [spark ignition] engine is a critical issue that needs addressing as pre-ignition may lead to engine knocking events that may damage engine catastrophically. … emanates from hot-spots inside the combustion chamber.”

Numerous sources provide additional information on this subject.

“Downsizing is an important concept to reduce fuel consumption as well as emissions of spark ignition engines. … engines are boosted … at high loads and low engine speed referred to as low-speed pre-ignition or LSPI. … the air/fuel mixture is ignited prior to the spark which results in the initial flame propagation quickly transforming into heavy engine knock. Very high pressure rise rates and peak cylinder pressures could exceed design pressure limits, which in turn could lead to degradation of the engine,” according to SAE Paper No. 2011-01-0342, “Engine Operating Condition and Gasoline Fuel Composition Effects on Low-Speed Pre-Ignition in High-Performance Spark Ignited Gasoline Engines.”

University of Wisconsin research reported, “Super-knock can suddenly damage the engine due to the extremely high peak pressure ... triggered by ‘hot-spot’ other than spark prior to the spark timing. … common knock suppression methods, including retard sparking timing … are not effective for super-knock.”

For the Customer — Who Doesn’t Hear Knock

Believe it or not, your customers won’t always hear their engines knocking.

According to SAE Paper No. 2014-01-1212, “Investigation on Pre-ignition and Super-Knock,” motorists may remain unaware of knock due to noise-reducing vehicle designs.

Super-Knock and The Solution

LSPI super-knock causes problems including broken pistons and bent connecting rods, with traffic safety hazards reportedly causing OEM abandonment of early designs. Service shops may still encounter vehicles with older designs that have been eliminated or updated by the manufacturer, and motorists may be unaware of knock damage.

The solution is to offer your customers preventive maintenance control of combustion chamber deposits that cause hot-spots and LSPI super-knock.

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