General automotive knowledge Q&A

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General automotive knowledge Q&A

Postby rburgy » Fri Aug 26, 2016 10:28 pm

Gentleman. I just recieved an article from one of the automotive insite websites
I belong to. This weeks issue was about carbon deposits causing running problems on VW and
AUDI direct injected vehicles. Also talked about camshafts wearing due to poor tempering of the camshafts.
Anyway, regarding the carbon deposits, it says that due to fuel being direct injected bypassing the intake valves
And having EGR- this is what caused the problem. Ok i understand that, it makes sense but, they also said these
Cars have variable valve timing on them too? Why would they need EGR with VVT? I always thought that was the
Purpose of VVT. Closing the exhaust valve a little earlier traps exhaust gasses in the cylinder- voila!! i personally do not see a lot of the cars here in Pittsburgh. Am i missing something???
Thats guys
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Re: General automotive knowledge Q&A

Postby andrewbishop66 » Sat Aug 27, 2016 7:44 pm

not really my field but vvt and egr are different

egr is used to reduce nox gasses and vvt to improve performance but as i understand they both have other side affects

Im sure someone can give a better answer

Thanks andrew
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Re: General automotive knowledge Q&A

Postby STC » Sat Aug 27, 2016 7:58 pm

This weeks issue was about carbon deposits causing running problems on VW and
AUDI direct injected vehicles. Also talked about camshafts wearing due to poor tempering of the camshafts.
Anyway, regarding the carbon deposits, it says that due to fuel being direct injected bypassing the intake valves


That article is about 10 years too late :D These have been common issues on these cars for yonks now !

I always thought that was the
Purpose of VVT. Closing the exhaust valve a little earlier traps exhaust gasses in the cylinder- voila!! Am i missing something???


Yes that is ONE purpose of VVT but by far not its only purpose

As you say with VVT it is possible to have some Auto Egr by playing with Valve opening & Closing times. Has been done for years now.

egr is used to reduce nox gasses and vvt to improve performance but as i understand they both have other side affects


The purpose of EGR as Andrew wrote is 1. to be a nuisance & 2. lower combustion temperature to prevent the formation of NO2, NO3 .... NOx as we all call them.

VVT is for Improved efficiency at different engine speeds & loads but some bright spark discovered it can be used for Auto EGR. The EGR Valve is fitted as well as VVT for more EGR, hence less NOx, than valve timing alone can achieve.

VVT (Auto EGR) plus EGR valve is still not enough to get NOx low enough to satisfy the emission targets. (VW Scandel)We can only have EGR on a warm Engine at low / moderate loads.

At times of high power demand, EGR will be closed & VVT may do the opposite with valve timing and encourage full excavation of exhaust gasses with an efficient inrush of cool air in to the chamber actually preventing any Auto EGR. Swirl Flaps, Manifold Design, Forced Induction .... .... ....

All the above Prevent NOx formation but not effectively enough, so now we have NOx Catalysts, SCR, Adblue ..... etc. to deal with the NOx formation that could not be prevented.
We have now come full circle with technology trying and failing with emission reduction, someone has now finally realised that all these nasty emissions are a result of burning fuel and by burning less we will get less emissions. That would be a big reason behind new cars now using smaller CC engines to manage emissions but have Superchargers and/or turbo's bolted on to satisfy performance requirements.
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