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Old 09-26-2018, 11:03 PM   #1
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Help with short bus engine

Hey guys,

I've started searching through these threads to find out what engine I should look for in my first short bus purchase. I don't know much about specific engines and what makes them good/bad, so I don't fully understand a lot of the information I've found (I'll learn). That being said, I have noticed there is a general consensus that diesel engines used to be really good (over 20 years ago), then got steadily worse, then something with emissions happened in like 2008 that made them untouchable.

I'm curious as to how meeting emissions requirements destroyed the engines to the point where people won't consider them. What is the major factor that changed in the engine around 2008? Is it more about power/speed/mpg type specs or is it a reliability/maintenance thing? Are the issues something that larger buses have more of a problem and short buses can deal with? Is a gas engine in a short bus better than a post-2008 diesel engine in a short bus?

It is also possible that I am way off-base and have misunderstood the information I have found, or I am missing some key information. If anyone could shed some light on this topic, I would greatly appreciate it.
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Old 09-26-2018, 11:17 PM   #2
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It's not what happened IN the engine, it's what else is bolted ON to the engine. The issue with the emissions is basically the exhaust - the exhaust needs to be cleaner (fewer emissions, less/smaller particulate matter.) In order to get cleaner exhaust, engine makers have done all sorts of funky things.


One of the solutions is to add DEF to the exhaust stream (Diesel exhaust fluid, or urea - a mild acid I believe) - to chemically help neutralize some of the emissions.

Another solution is sometimes to re-circulate some of the exhaust gasses back through the engine, helping it burn hotter, to get a more complete combustion cycle.

All of these extra systems are just that - extras. More parts, more hoses, more complicated routes for hoses and lines to snake across and engine bay. There's more "stuff" to break. Some of these systems also have complicated sensors (and engine computers), which are just by nature harder to tweak/adjust - you can't just turn a screw or replace a spring to dial up the power a little bit. Some of the old engines would literally run until they ran out of oil, or some imbalance shook the innards of the engine apart and sheared something apart - the modern engines will shut themselves down or go into limp mode if the detect a fault with a sensor.


In some ways, you could summarize an old diesel engine this way :



Fresh air in the front -> Heat/noise -> POWER!!!! -> Exhaust out the back

Now its more of a giant squiggly line with computers, sensors, DEF, EGR valves, mufflers, more computers, and a whole bunch of wires.



I don't think any manufacturer has come up with a particularly good system - I'd equate it to the early CFL light bulbs from 10 years ago when people began to think about being "energy efficient." A lamp is a lamp, that hasn't changed - but the bulb has. CFLs took forever to warm up (and get to full brightness), they got dimmer over time, didn't like the cold, and the color temperature of the light itself was often pretty bad. Forget using a dimmer. Now we have LEDs, which are cheaper, sturdier, and provide light that looks just like a standard light bulb, and cost only slightly more.
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Old 09-27-2018, 08:10 PM   #3
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Thanks for the reply that makes a lot of sense!
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Old 09-27-2018, 08:34 PM   #4
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When those issues raise their ugly head they are very time consuming and expensive to repair. Issues started around 2005
The issue doesn't care if it's a short or long bus.
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Old 09-27-2018, 09:10 PM   #5
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There are basically three different emissions components that are installed.

Before we get into that, there are basically two emissions that are being controlled: Soot or particulate - the stuff you can see and NO2 - basically poison - nitrogen dioxide.

Soot is created when there is a low burn temperature and not all of the fuel is burned completely.

NO2 is created in opposite conditions, when the combustion is too high.

In 2004, a PCV or Pollution Control Valve was required. This basically took the soot and put it back into the fuel-air mixture. As you can guess, this burned the unburnt fuel/soot. But it also made the combustion less efficient than clean outside air.

The next thing that was introduced in 2007 was the re-burner. Basically this is like a catalytic converter. This injected extra fuel into the combustion chamber causing a richer/hotter burn. The soot trapped in the re-burner was then catalyzed and burned off. This however caused NO2 - deadly poison.

The latest diesel emissions control to be introduced is DEF or Diesel Exhaust Fluid, or urea. A fancy name for pig urine with a 30% urea concentration. Normal urine is 1.5-2% so no, you can't pee in the DEF tank. Basically, urea bonds with NO2 and the output is nitrogen and water - cool, but problematic. DEF needs to be injected as a fine mist. If DEF/Urea sits too long, it can crystalize in the injectors and cause it not to work - and DEF injector replacements are expensive.

So, as you can see each of these measures are problematic, but they do the job as designed. Unfortunately, the PCV and the re-burner take away fuel economy and HP from the engine.
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