The Atkinson Cycle engine

[Neil Ives]

Apologies if you've already seen this.
https://www.autodeal.com.ph/articles/car-features/what-atkinson-cycle-engine-and-why-it-fuel-efficient

[nowster]

https://en.wikipedia.org/wiki/Atkinson_cycle#Modern_Atkinson-cycle_engines

In the late 20th century, the term "Atkinson cycle" began to be used to describe a modified Otto-cycle engine—in which the intake valve is held open longer than normal, allowing a reverse flow of intake air into the intake manifold. This "simulated" Atkinson cycle is most notably used in the Toyota 1NZ-FXE engine from the early Prius.

The effective compression ratio is reduced—for the time the air is escaping the cylinder freely rather than being compressed—but the expansion ratio is unchanged (i.e., the compression ratio is smaller than the expansion ratio). The goal of the modern Atkinson cycle is to make the pressure in the combustion chamber at the end of the power stroke equal to atmospheric pressure. When this occurs, all available energy has been obtained from the combustion process. For any given portion of air, the greater expansion ratio converts more energy from heat to useful mechanical energy—meaning the engine is more efficient.

The disadvantage of the four-stroke Atkinson-cycle engine versus the more common Otto-cycle engine is reduced power density. Due to a smaller portion of the compression stroke being devoted to compressing the intake air, an Atkinson-cycle engine does not take in as much air as would a similarly designed and sized Otto-cycle engine.

[John Ratsey]

The lower torque and power are particularly noticeable on the Mk 3 Jazz which has an Atkinson cycle mode up to about 3000 rpm resulting in lethargic performance at lower revs but above that speed different valve operation made the vehicle more enthusiastic. On the Mk 4 Jazz the electric drive provides the torque and makes the deficiencies of the Atkinson cycle irrelevant.

[grahamsmithbsc]

Does direct injection cause inlet valve carbon buildup after 60,000 miles or does the Atkinson Cycle mitigate this problem by keeping the valve open on the compression stroke?

[Jazzik]

Your Jazz (Mk3) has multipoint injection, not direct injection...

[grahamsmithbsc]

The Atkinson Cycle engine is fitted to the Mark 4 Jazz Hybrid.

[JB]

Does direct injection cause inlet valve carbon buildup after 60,000 miles or does the Atkinson Cycle mitigate this problem by keeping the valve open on the compression stroke?

Yes direct injection will cause carbon build up, I know that from experience with a vauxhall
at 90,000 miles , it has to be for me either port or Muti-point injection in future, direct injection ok
till you get some milage on the clock. as Jazzik posted jazz is Muti-point.

[grahamsmithbsc]

Agreed. I found this.
Atkinson cycle engines, commonly used in hybrids, are prone to inlet valve fouling (carbon buildup)
due to their frequent use of direct injection and low-temperature operation. The late closing of intake valves can cause backflow, pushing oil vapors and exhaust gas residue onto the valves, where they bake into hard deposits.
Pity. I bought Honda to avoid these long term problems, so dissapointed.

[Lord Voltermore]

Agreed. I found this.
Atkinson cycle engines, commonly used in hybrids, are prone to inlet valve fouling (carbon buildup)
due to their frequent use of direct injection and low-temperature operation. The late closing of intake valves can cause backflow, pushing oil vapors and exhaust gas residue onto the valves, where they bake into hard deposits.
Pity. I bought Honda to avoid these long term problems, so dissapointed.

This does NOT  mean that the  Atkinson cycle engine in the Jazz mk4  is one of those that uses direct injection .

The following is pasted from the specification of the Jazz mk 4 engine as listed by google AI.  Although you cant always trust AI  it does list the specifications in detail.  Text colour changes by me. 

Engine Technical Specifications (1.5L e:HEV)
Engine Type: Inline-4, Atkinson-cycle DOHC i-VTEC
Engine Code: LEB8
Displacement: 1498 cm³
Max Engine Power (Pre-facelift): 98 PS (72 kW) at 5500-6400 rpm
Max Engine Torque: 127 Nm (13 kg⋅m) at 4500-5000 rpm
System Combined Power: 109 PS (80 kW) / 107 HP
Updated System Power (2023+): 122 PS (120 bhp)
Combined System Torque: 253 Nm (187 lb-ft)
Compression Ratio: 13.5:1
Fuel System: Multi-point injection
Thermal Efficiency: 40.5%

 

[Nicksey]

So, in a nutshell..

The Atkinson unit in the Jazz is a highly efficient and reliable engine that does not suffer from carbon build up. 

[Jazzik]

Fuel System: Multi-point injection

Pity. I bought Honda to avoid these long term problems, so dissapointed.

Dissapointed? With multi-point injection?

[grahamsmithbsc]

If it is multi-point injection in my eHEV then I'm more than happy as, barring accidents, I'm counting on this car to last me out. By far the best car I've ever owned.

[grahamsmithbsc]

I notice the output power was uprated in 2023. I haven't seen this before?

[Lord Voltermore]

I have read before  that the 2023+ facelift has increased  power . But I havnt really noticed it in any significant increase in  road performance between my 2021 and 2024 cars  to the extent you would expect from such a large increase in a car with a conventional engine/drive system.    And no increase in insurance.

For a while I thought it might only apply to the sports version introduced at the same time.  But I now think both share the same specification.

But there is a more subtle difference. I find the facelift is slightly  quieter and more refined than before.  I think the extra power has been used to increase the recharging  performance  of the HV battery rather than increasing road performance. This  allows the electric drive to contribute a bit more towards the overall  refined experience of hybrid drive.  Both cars still claim the same, very respectable ,0-62mph/100 kph time of 9.4 seconds - which can be  reduced by a second if the HV battery is fully charged.)  The difference is not enough to dismiss buying a 2020 -2022 car  IMO

[stani]

This does NOT  mean that the  Atkinson cycle engine in the Jazz mk4  is one of those that uses direct injection .

That's right, the current Jazz doesn't have direct injection.

But in the future, the next generation will probably have direct injection, there was a repeated reference to new Honda engines in the forum.