Gas & diesel engines are converging

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Gasoline/petrol and diesel engines appear to be converging in their technologies. Gas engines are adopting diesel techniques like direct injection, lean operation, higher MPGs (40+) while diesels are moving towards low compression, richer burns, and lowering MPGs. (Example: Jetta TDI dropped 45 downto 42.)

Example from Mazda engines:
- SkyActive G-motor is 14:1 (highest compression for a gas engine)
- SkyActive CDI engine is 14:1 (lowest compression for a diesel)
They are almost identical in design, except one is spark ignition & the other is not. Cool. :-D
 
Uh identical being that the diesel has 2 turbos? Seriously?

And the jetta also grew in size and has to run ULSD now and has more strict emissions requirements... not to mention the change in MPG measurement in 2008.
 
I didn't realize the CDI had 2 turbos. I guess that helps achieve the ultra-low compression? Also the numbers quoted for the older Jetta ARE the new test results, because the original EPA rating was 49 highway (and then revised to 45).
 
Lol. This is not a case of technologies evolving naturally. This is a case of goverment mandates fprcing cars to be less efficient for the sake of being cleaner.
 
One of the main reasons diesels typically use such high compression is for cold starts - the higher the CR, the more the temperature in the combustion chamber increases when the piston moves from BDC to TDC on the compression stroke. Mazda had to do some interesting things with injectors and valve timing to get reliable cold starting with low CR.
 
The degradation in diesels is almost entirely emissions-driven. Once the manufacturers figure out the best way to orchestrate particulate filtration, aftertreatment, and EGR I hope (knocking wood) that diesel specific fuel consumptions will drop back down and continue to exceed spark-ignition. Given what actually goes on in the little chemical refinery that is a modern diesel exhaust system, I'm surprised they work at all, let alone work as well as they do. It'll get better.

But in some ways you're right- DI on the gasoline side at least opens the door to a true "all-fuels" engine of the type that was being researched by Ford and Texaco (among others) back in the early 80s. At that time, a real stumbling block was fuel pumps and injectors that could direct-inject very low-viscosity low-lubricity fuels like gasoline and alcohols and hold up over the long term. But that's obviously sorted out now, and its emissions, sensors, and engine management required to operate on such a wide range of fuels. Imagine what would have to go on with the engine management system if a driver started with a tank of diesel, filled up with gasoline at 3/8 of a tank, and then bought E85 when the needle read 1/4 tank! It CAN be done, but right now the market isn't there to support the required cost.
 
Originally Posted By: sdowney717
Even if such an engine existed, people would soon figure out what was the cheapest fuel that gave the highest mpg and use only that.


Which would (eventually) drive fuel to be priced per BTU instead of per gallon. It would take more gallons of gasoline to equal the same BTU content in diesel, but the cost to the consumer would be the same. Which would ultimately allow more effective use of crude fuels (including crude oil, unprocessed recycled oil, unprocessed bio-oils, etc.)
 
If you compare the BTU/mile of a modern Jetta diesel vs. Jetta gasoline, they are almost identical (~3150). Diesel has lost its efficiency advantage.

And in terms of cost, it's a tossup which is cheaper to operate..... I would guess the gasoline model, since diesel often costs the same as supreme.
 
Originally Posted By: NateDN10
One of the main reasons diesels typically use such high compression is for cold starts - the higher the CR, the more the temperature in the combustion chamber increases when the piston moves from BDC to TDC on the compression stroke. Mazda had to do some interesting things with injectors and valve timing to get reliable cold starting with low CR.


Nope, C.R. equals thermal efficiency...that's a diesel fundamental.
 
Mazda says their low-compression diesel is More efficient (versus their standard H.C. diesel) wwith more thorough burning of the fuel & less soot produced. They were also able to increase the redline to almost 6000 rpm, like a gasser
 
I often wonder, if gassers were required to have the same emissions control devices on them that the current crop of diesels have, how the public would react. Imagine.... soot filters like DPF's on diesels, having to add Urea based products for SCR units, etc. And the associated costs when these systems take a dump.

I am convinced that the lower compression ratios being tried out in diesels has more to do with getting the engine to a level that would facilitate eliminating the EGR systems on them. This would reduce NOx levels to where the SCR could be the single solution, and it would reduce soot levels that have been a big part of EGR use on diesels so that the particulate filters last longer and have longer regen intervals. Once everything is moved off of the engine, and all emissions stuff is kept downstream, engines would have greatly reduced maintenance issues.
 
Originally Posted By: blackman777
Mazda says their low-compression diesel is More efficient (versus their standard H.C. diesel) wwith more thorough burning of the fuel & less soot produced. They were also able to increase the redline to almost 6000 rpm, like a gasser

Yeah, Mazda bragged about their new technology making a 5300 RPM rev limit possible, that can't exactly be true. They said their 14:1 compression ratio made it possible.

Back in the 80s, there were numerous diesels in cars that revved to about 5000 RPM, and they had about 21:1 compression, because engines using pre-chambers needed extra compression.
 
Originally Posted By: blackman777
Mazda says their low-compression diesel is More efficient (versus their standard H.C. diesel) wwith more thorough burning of the fuel & less soot produced. They were also able to increase the redline to almost 6000 rpm, like a gasser


the basic thermodynamic cycle that IS the diesel cycle increases efficiency by increasing the compression ratio...yes, less soot is "more efficient" than more soot, but not to the degree that compression drives efficiency.

The reduction in soot is a design specific, and soot reduction can be applied to a diesel of high compression also...similarly, compression doesn't change redline...my 3.0L turbodiesel Nissan will pull 4,200 RPM (redline is 4,500 but governor won't let it go there)

TiredTrucker, IMO is spot on that they are trying to reduce NOx with lower compression.
 
Just put 15 gallons of urea in my friends giant diesel pusher motorhome this weekend during an oil change. 15 quarts of oil, too!

IMO in cars we haven't yet found the right attributes for a diesel. When they become completely like a gasoline powered engine in all ways then we might look at one. I love mazda's focus on the driving experience, it represents a step in the right direction IMO.

Noise, vibration, smells, frequent services, not yet for us....
 
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Just changed the in tank fuel pump on my L67 Caprice yesterday (and the blower coupler), which I've never had to do on my diesel Nissan ute.

My 10 year old Nissan ute (main family car) uses 30% less fuel (volumetricaly) than the gasser, using a fuel that is about 6% more cost per litre than E10...savings are more wehn we are in the big smoke, as the diesel doesn't drop as much mileage...250km more range on a trip is certainly handy...10k services for both diesel and gasser.

Will put the second set of glowplugs into the machine probably in Autumn now, as with the warmer weather, the gasser buyers don't have to do that admittedly, but they have more coils and leads.

Do you honestly think that the EPA (or whoever) would require Urea for a gasoline engine, when they are "mainstream", not a fringe dweller that can be laughed and pointed at ?

Lean burn D.I. gassers will be in the same Nox wagon as diesels, and I'll bet you a dollar that they are NOT treated the same.
 
Indeed, if you are correct that will be strictly due to our political climate and the oil companies big expenditures.

But they are coming along (diesels), I already have looked at the new jeep grand cherokee with a diesel and it was a beaut.

Note that regular here is 2.98, premium is 3.39 and diesel is 3.71.

They will need a big advantage somewhere over a similar powered gas vehicle to get our dollars. Plus they simply do not work in our fleet due to there not being enough heat generated at low loading in stationary operations...
 
There are ways around the heat issues....in our bus fleets (and locomotives too), diesel-fired coolant heaters are being used as supplementary heat. The heaters are used as engine pre-heaters and once the engine is started, the heaters are thermostatically-controlled to maintain engine heat at the ideal 180'F. We use the heaters in our heavy-equipment fleet as well but only as pre-heaters.

While on topic, we drive a BMW 35d on our 150 mile round trip commute. 28MPG, ability to navigate any road condition we might encounter, and the comfort/protection of a large luxury vehicle make it worth my while to purchase diesel fuel and urea. I change our oil every 5k miles (contrary to BMW's 11k interval) and add about 1 gallon of urea. We get around 4 MPG to 8 MPG (depending on engine size) better than comparable X5's with gasoline engines on the open road and our commute, although 90% highway, is full of steep hills and stop-n-go traffic on the way home for the first 45 minutes.

I acknowledge though that diesels are not for everyone and DPF's and EGR's are a little scary.
 
Glad to know they work for you. Our factory authorized aftermarket upfitter has a 3rd world version that uses a diesel burning heater setup but it adds a whole layer of complexity and failure points that we neither want or need. Thermostats, complex plumbing with bypasses, safety systems to prevent steam explosions, etc.

Our gasoline engines use a simple stainless box with copper coils inside that takes coolant before the heater core and easily produces huge volumes of perfectly heated water while cooling the engine extremely effectively during stationary ops. Very simple and highly effective.

42 years in this biz has proven to my satisfaction that simpler is indeed better over the long haul...
 
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