Why do diesels run Pyrometers and gas engines don't?
07-29-2006, 09:54 AM
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Car: 1989 Firebird
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Why do diesels run Pyrometers and gas engines don't?
Just talking with my dad about turbo's and this question came up. I didn't have an answer 
Anybody know why?
Anybody know why? 
07-29-2006, 10:03 AM
#2
while EGT gauges aren't near as common on performance gasoline engines they are used. Just a theory on why they're more prevelent on diesels is that soot contaminants would eat up an O2 sensor quickly.
07-29-2006, 10:43 AM
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Originally Posted by Drac0nic
while EGT gauges aren't near as common on performance gasoline engines they are used. Just a theory on why they're more prevelent on diesels is that soot contaminants would eat up an O2 sensor quickly.
Sorry for the 20 questions
07-29-2006, 11:38 AM
#4
Originally Posted by crazyjim
I don't get it, what's so detrimental about an exhaust getting hot? Why would it matter with soot and 02 sensors. Pyrometers measure heat not soot so it's going to plug up the sensor anyway isn't it? Where are Pyrometers located?
Sorry for the 20 questions
Sorry for the 20 questions
O2 sensors are sensitive devices, it's not that difficult to kill one. They don't like excessive amounts of unburnt fuel, particulates and petroleum products. This is where an EGT system is better used.
Another thing is that diesels tend to have fuel and air crammed into them as long as it's not getting excessively hot. That's why if you look at a lot of diesel products they tend to push "lower EGTs" as a selling point.
07-29-2006, 06:23 PM
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Originally Posted by Drac0nic
er, heat turns engines, turbos and other parts into slag. slag=bad. The reason you use a pyrometer in a gasoline engine is the same reason you use an O2 sensor, you get an indication of the AFR based on the exhaust temperature.
O2 sensors are sensitive devices, it's not that difficult to kill one. They don't like excessive amounts of unburnt fuel, particulates and petroleum products. This is where an EGT system is better used.
Another thing is that diesels tend to have fuel and air crammed into them as long as it's not getting excessively hot. That's why if you look at a lot of diesel products they tend to push "lower EGTs" as a selling point.
O2 sensors are sensitive devices, it's not that difficult to kill one. They don't like excessive amounts of unburnt fuel, particulates and petroleum products. This is where an EGT system is better used.
Another thing is that diesels tend to have fuel and air crammed into them as long as it's not getting excessively hot. That's why if you look at a lot of diesel products they tend to push "lower EGTs" as a selling point.
07-29-2006, 09:41 PM
#6
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To be alittle clearer:
On a spark ignited (gas or SI) engine your throttle/accelerator controls an air/fuel mixture or an "air limited" mixture (ie: throttle or butterfly valve). Simply put a "rich" mixture is considered safe (although not necessarily best for power, economy, or emissions). Too "lean" a mixture can result in excessive heat and burn an valve or piston.
On a compression ignited (diesel or CI) engine the accelerator controls the only the amount of fuel. There is no throttle or control of the amount of air ingested. The air/fuel mixture varies greatly depending upon load/RPM. A fuel "rich mixture is more correctly referred to over-fueling and is noted by a black sooty exhaust. Exhaust temperatures increase greatly at this range, and approach the physical limits of the engine (exh valves in particular).
Now before I get sharp-shot there is ALOT of techinical info I left by the way side here: burn properties of diesel vs. gasoline, cetane and octane ratings, direct injection (diesel) vs. manifold injection (gas), turbocharging supercharging, and odd-ball Mercedes throttling injection pumps, etc. BUT in a nutshell this is why you see pyrometers on diesels and AFR on gas. Pyrometers do work on gas engines but their results are harder to interpret on a non-steady state engine. -You do see most small (light) airplanes with pyrometers, as the pilot controls the air/fuel mixture during operation.
On a spark ignited (gas or SI) engine your throttle/accelerator controls an air/fuel mixture or an "air limited" mixture (ie: throttle or butterfly valve). Simply put a "rich" mixture is considered safe (although not necessarily best for power, economy, or emissions). Too "lean" a mixture can result in excessive heat and burn an valve or piston.
On a compression ignited (diesel or CI) engine the accelerator controls the only the amount of fuel. There is no throttle or control of the amount of air ingested. The air/fuel mixture varies greatly depending upon load/RPM. A fuel "rich mixture is more correctly referred to over-fueling and is noted by a black sooty exhaust. Exhaust temperatures increase greatly at this range, and approach the physical limits of the engine (exh valves in particular).
Now before I get sharp-shot there is ALOT of techinical info I left by the way side here: burn properties of diesel vs. gasoline, cetane and octane ratings, direct injection (diesel) vs. manifold injection (gas), turbocharging supercharging, and odd-ball Mercedes throttling injection pumps, etc. BUT in a nutshell this is why you see pyrometers on diesels and AFR on gas. Pyrometers do work on gas engines but their results are harder to interpret on a non-steady state engine. -You do see most small (light) airplanes with pyrometers, as the pilot controls the air/fuel mixture during operation.
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