Automotive supplier Bosch says it had nothing to do with Volkswagen's use of "defeat device" software to cheat on emissions tests.

Almost immediately after news of the scandal broke in September, it emerged that Bosch had supplied some components in the affected TDI diesel models, including control software.

But Bosch says individual carmakers bear full responsibility for complying with emissions standards.

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While putting the blame on Volkswagen, Bosch CEO Volkmar Denner defended diesels in a press briefing last week in Germany, according to Bloomberg.

He said diesels are "air cleaning machines," with emissions that are less dirty than the air they take in.

As a group, diesels produce roughly 20 percent less carbon dioxide than gasoline engines of similar output--and if emissions controls like particulate filters and Selective Catalytic Reduction (SCR) systems are employed, they comply with all current emission regulations in North America and elsewhere.

2015 Volkswagen Golf TDI SE

2015 Volkswagen Golf TDI SE

SCR systems--which inject urea fluid to clean exhaust--were not installed on many of the affected VW TDI cars.

Because of the amount of hardware involved, retrofitting cars would be extremely complex, and could cost thousands of dollars per car.

Instead, VW installed software that could detect the conditions of a laboratory emissions tests, and temporarily adjust engine operation to lower emissions during those tests.

ALSO SEE: New Investigations Of Bosch, EU Loans In VW Diesel Emission Scandal (Dec 2015)

Many engineers at other car companies initially thought it was impossible for Volkswagen to meet U.S. emissions standards without SCR--and they turned out to be right.

This has led to questions from analysts about whether it will be practical to modify cars to meet emissions standards, and whether VW should just buy at least some of the cars back.

Bosch maintains it wasn't involved in any of this, but officials are still double checking.

2015 Volkswagen Golf TDI

2015 Volkswagen Golf TDI

CEO Denner said Bosch opened its own internal investigation the day after Volkswagen's diesel cheating was revealed in a press conference held by the U.S. Environmental Protection Agency.

Bosch is also being investigated by U.S. and German authorities to see whether it helped Volkswagen cheat emissions tests.

MORE: BMW Admits VW Emission Scandal Hurts All Diesel Sales In U.S.

The company is supplying more documentation to investigators than simply what was requested, Denner said, to help provide a full technical understanding of the matter.

He said Bosch hasn't noticed a drop in demand for diesel cars, but believes continued intense scrutiny by authorities could eventually taint diesel's reputation.


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